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Head unit wiring questions!


24 Jan 2019
One of the projects I've embarked on recently is to upgrade my in-car entertainment. I have an '04 C4S, which comes with the Bose Fibre Optic MOST system. The car is not optioned with PCM2, so it has a single din CRD-23 radio/cd unit. It does not have a CD changer. The amplifier is on the passenger side of the boot compartment (visible in the compartment next to the spare tire under the boot floor.

I do not plan to change the amplifier or re-wire any speakers. Instead I will use the existing fibre optic system and use a MOST HUR converter (already purchased). I'm aware of the limitations of this with respect to fade etc. The converter will convert an analogue RCA-plug stereo signal from the back of my new head unit to the digital signal for the existing amplifier.

I have purchased:

  • - Sony XAV-3005DB double din CarPlay/android auto head unit with DAB
    - MOST HUR converter
    - New bottom shelf faceplate for HVAC unit
    - New cubby shelf unit to go above HVAC unit.
    - Double din cage / fitting kit.
    - An ISO harness which I think will basically be useless, but may provide some connectors I can re-purpose.

So far I have moved the HVAC unit down from the top slot in the console to the bottom slot. I have removed the CDR-23 unit, the CD-holder unit and the cubby. This leaves room in the top two slots for a double din unit. However, because I have the single din setup on my car I have had to remove the Centre Bezel (996-552-233-06), unplug all the switches, and remove the support frame (996-552-231-110-1C) that has a plastic horizontal brace that will need to be cut away. I'm not too concerned about doing this (they're about £45 if I want to put it right in the future). I could buy one for PCM2, but I'm not sure if a double din unit would fit in that without modification anyway...

What I do need help with is figuring out all of the wiring. I have found lots of threads and information on the internet, but not a single guide that explains the best approach for my car/configuration.

Here's what I have in my car coming out of the back of the CDR-23 unit...

A fibre optic plug. Easy... This goes into the output of the MOST HUR converter.

A black plug. This plug has eight pins, three of which are vacant. Here is what I *think* they mean. Please correct if wrong:

  • pin 1 - pink/grey (SPEED VOLUME?) I see 12v when key ignition on
    pin 2 - Black/blue (REVERSE?) I get not voltage even when car running in reverse
    pin 3 -
    pin 4 - red/black (CONST POWER?) I get constant 12v from this
    pin 5 - white (AMPLIFIED ANTENNA?) I get nothing ever. Is it an output?
    pin 6 -
    pin 7 -
    pin 8 - brown (GROUND?)

I was surprised that the reverse wire doesn't send any signal when the car is in reverse. Maybe none of the cars needed that.

There is also a small yellow plug. This plug has six pins, two of which are vacant. Here is what I *think* they mean:

  • pin 1 - yellow (CAN COMFORT HIGH??) no voltage ever
    pin 2 - black (CAN COMFORT LOW??) constant 4.8-5v volts
    pin 3
    pin 4 - red/white (WAKE UP DIAGNOSE??). constant 12v signal
    pin 5 -
    pin 6 - green (KEY CODE IN??) constant 12v signal.

I don't really understand anything about the yellow plug. What does the CDR-23 use the canbus for? I'm assuming my new stereo doesn't need any of these wires but I'm not sure.... I could do with some guidance here...

OK, so my MOST HUR converter box will need the following:

  • wire 1 - black (ground)
    wire 2 - yellow (const 12v)
    wire 3 - red (acc 12v)

I think this makes sense. It wasn't both a constant signal and an accessory one (one that is 12v when the ignition is on). I guess this is so that it only powers on when the stereo is actually in use? Given that... am I right in thinking that the acc red signal should actually come as an *output* of the stereo?? That way, the box activates when the stereo is on?

My Sony head-unit has the following output connections:

  • pin 12 - yellow (CONT POWER)
    pin 13 - blue/white (POWER AMP / AERIAL REM)
    pin 14 - orange/white (SWITCHED ILLUMINATION)
    pin 15 - red (SWITCHED POWER)
    pin 16 - black (GROUND)

The black plug from existing CDR-23 unit has a ground wire and a constant power wire. Great.

Question: I don't really understand what pin 13 is for, but my assumption is that that pin goes 12v when the stereo is switched on? That way you would power you electronic aerial (if your car has one, or your amplifier) only when you turned the stereo on. Is that thinking correct? If so I think that the blue/white cable would need to go to the red acc wire of the MOST HUR converter.

Next question: what about the amplifier? I don't know where I need to send power to the amplifier. Anybody got any idea about that? At a guess I would wonder if it's the white cable on the black plug of the CDR-23? Pin5 on the black plug...

That leaves two remaining wires on the Sony to hook up:

- red (SWITCHED POWER). Where can I get an accessory power source from? It needs to be something that's only live when the ignition is on. What do people use for this? I would need to connect the Sony to that source right?

- orange/white (SWITCHED ILLUMINATION). I think that this needs to be something that only has a signal when the headlights are switched on. The nearest place I think I can take that from is the grey/blue wire on the cigarette light to the left of the stereo. This powers the ring light around the lighter. Is this where most people take switched illumination from??

Final question! Why does the head-unit need a switched power (red acc) input if it has a constant power (yellow) input? If it has constant power, why can't it just use that to power itself on and off? I guess I don't understand the point of the head-unit needing both a constant power and an accessory power?

Thanks for any input. I have various pictures if that helps at all...
I believe you have there part Of The mini iso block (yellow) and the iso block connector. A photo Of The connection and I can confirm 100%.
See https://en.wikipedia.org/wiki/Connectors_for_car_audio

Mini iso is covered in there too with wiring, although IIRC a couple of the pins are different/swopped on the 996 blocks to what's listed there.
Hi Asterix,

I have attached a couple of photos. You're correct. Every piece of documentation I have seen refers to the yellow plug as C1. Indeed, the position in the back of the radio suggests it's in position C1.

That link you pasted seems to make a lot of sense. It seems the information I found on the C1 plug (with connections being for canbus etc) is wrong then? I think I got they from renntech or somewhere...

According to that wiki page, pin 6 is supposed to be a switched power signal. Is that to come from the stereo when it's switched on, to power the external amplifier?? If that's the case I'm surprised that it had constant power when I tested it (as if it was a constant input).

Maybe, as you say, some of the wires are changed around on the 911?


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Thanks deMort. They look significantly different though (my car is the generation of PCM2, though it wasn't optioned on my car..)

For example I don't have an illumination connection (I guess because there is nothing to dim or put in night mode on the CDR-23), so I have to pick that up somewhere. And for some reason I don't think I have, or at least I can't find, an acc (accessory) connection.
Thanks deMort.

On another note, I noticed a few folks on various other forums suggesting that all 996s have the telephone prep cable as part of the main harness.

This would be excellent as there would be an illumination and a switched power (acc) on there.

Alas I have had a proper rummage this morning (everything is currently out of the mid console) and I could not see this darn telephone prep cable hiding anywhere.

So I guess by 2004 they had not bothered to make the cable part of the harness. I suppose that makes sense given how silly those phones were...

So, still looking to take illumination from cigarette lighter ring, and then switched power from somewhere else... Input welcome.
So I managed to get everything working! Did it all with everything hanging out of the centre console, so to speak..

I didn't look for a switched power (I just connected switched inputs to a constant power source). This was just for testing. I was more worried the chinese MOST converter wouldn't work etc.

But everything did!

In terms of switched power I have decided to go from the fuse board with a piggy back fuse and thread a switched power supply through to head unit. I don't want to start tapping into the HVAC cable harness for switched power.

Does anyone know of a possibly empty fuse that's switched? Failing that I'll piggy back one, e.g., the rear wiper or something.

I'm planning on doing a write up as I basically had to figure out almost everything for this. None of the existing forum threads seems to be right for a single-din CDR-23 on a C4S.
So I completed the install. Pleased with it. I'm going to try and find time to a guide for people. Hopefully it will be useful.
Heed - yes please; can you add it on to below this thread for continuity...??

I ripped out my old system 2 years ago and changed for an iPad but i'm getting fed up with it. Intending to swap this out in the near future for an Android unit and refit the MOST so i can use the screen antenna, etc.

Would love to see your guide please :thumb:


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