I am new to this forum and new as an owner of a 2004 P12 1.8 litre petrol wagon. I've searched my way through this fine database of ours and didn't find anything like this.

My radio is unable to find stations (nothing even when I manually input a known good broadcasting station). The CD does work, but the issue with audio playback is horrendous quality. 

Upon starting the car I can hear the speakers pop quite alarmingly, pointing to something going on with amplification. After this the music comes on very muffled and distorted. The level of muffling and distortion varies on its own (meaning that the engine rpm has no effect). After x time elapsed, the audio volume and quality equalizes and clears up to a comprehensible state.

One curiosity I discovered is that the loudness setting does absolutely nothing. Also the max volume comfortable to listen is around 5 green bars with a very steep curve between steps of volume.

Things I've tried:

- Charge the battery

- Check battery terminal tightness

- Checked fuses to be OK

- Simple earth fix on both rear lights

- Cleaning and refitting of B-pillar ground points (M17-18 i believe?)

Things yet to try:

- Grounding of the cars aerial, which's ground point I've yet to discover (even from the manual)

- Reconnection of the CD player (due to the ratio of my understanding of centre console trim required to remove and time to complete such a feat)

- ???

It isn't new and it sure isn't perfect in other areas either (problems that probably shouldn't be discussed in this post).

The thing has had two error codes pop up (multi cyl misfire prior to clearing the codes and cyl 4 misfire after). There are two main problems with the engine at the time, which are 

- running/accelerating jerkily

- having a hard time starting randomly,

neither of which have anything to do with the engine being warmed up/cold.

It's good that you have taken the time t research the forum.

As you have found out earths are very important om this car, I would continue doing what you are doing in particular the aerial earth although to me it sounds like a capacitor broblen on the audio board.

As for you starting and jerking problems and it may just be be linked to the audio but  doubt it, check the two main battery earth leads, one to the chassis the other to the transmission, ensure good solid connections and no breaks in the cables.

A replacement unit would have to be re-coded by the dealer unless you got an expensive replacement from clarion. It has been done the way you describe with an aftermarket unit.

You may be able to get the Radio repaired by Clarion Service. Probably not worth it.

http://www.clarion-service.co.uk/cs/Home.html" rel="nofollow - http://www.clarion-service.co.uk/cs/Home.html

Aftermarket is an option below the Factory Radio. Requires some cutting of the Console.

Your Engine may have a stretched Timing Chain with those symptoms.

That probably needs investigating before spending money on the Audio.

No harm in swapping the Plugs/Coil around. I believe the Coils do sometimes fail.

Not really my area but I recognised the Chain symptoms from other Reports on here.

I assume you are contemplating on which yellow wire to use for your BAT/constant 12v connection. You can use either one for it. I guess the original headunit has two connections to be able to pass through power for all the other optional electronics available officially (sat-nav, subwoofer, cd-changer).

Now in terms of hooking up to these wires what I did was the following. Cut the cables for ACC and BAT, leaving ~5cm of cable to the connector end. Crimp on male 6.3mm flat insulated connectors to the connector end, and 6.3mm insulated piggyback connectors to the loom end. This way you'll be able to power the original unit and have connections available for your aftermarket one. 

Then crimp female 6.3mm flat insulated connectors onto the corresponding wires of your aftermarket headunit (constant 12v and ignition power) which you can connect to the spare ones on the piggyback connectors. Using the types of connectors I did you are going to want to protect the ACC/BAT connections from shorting to bare metal. I used insulation/duct tape, you can use anything that suites you. I've attached a picture of these connections below.

For ground you can use the metal supports of the centre console, see image below. Available points are on either side of the trimless centre console, I used the left hand side one.

Images for reference: