I've had a good trawl through the forums and I think I've found the answer I'm looking for, but before I try to fix it I thought I'd sound this out to see if I'm right.
In short I've recently bought an 8L A3 1.6 ('99) which is in good nick, but had a standard Concert HU (non-BOSE) and standard amp and subwoofer fitted which had the common volume problem making it unusable, so as a fix the garage fitted an aftermarket Sony HU (CDX-GT252MP) which on the face of it is all good. However after a bit of fiddling about with the settings it is clear that the rear speakers are much louder than the front, so to rectify I have to fade the a lot to the front to balance out.
Thinking this was wrong I pulled the HU out to look at the wiring and I saw what had been done. The fitter hadn't used an adapter, but had snipped and joined wires themselves. Now after reading a number of posts on here (largely detailed by AndyMac) I read that the wires on the Yellow/Green/Blue connector block, the yellow part of which I understand are the wires for the amplified rear speakers, had been snipped and wired directly into the rear speaker out of the HU.
This to me looks wrong as the HU will be outputting at speaker levels on those wires instead of connecting it to the RCA out, which will have a lower level output ready to feed the amp and hence balance out the sound in the car.
What they should have done (I'm assuming) is use an adapter, commonly this looks to me to be the PC9-404, which would have had all the right bits in the right place.
So my thinking is to either rewire these outputs to some RCA plugs and stick it in the RCA out of the HU. Either that or connect back up the yellow block, get a PC9-404 and, plumb that all back in. In fact I've got a Pioneer HU which I'd rather have anyway, which I have a Pioneer 16 pin-ISO connector for already, so I would probably whip the Sony out and put the Pioneer in instead.
I accept that the standard amp/sub/speakers aren't the best in the world by a long shot, but I'm not too worried about that. I just want to be able to play some music and listen to the radio clearly (and loudly when I feel like it).
Here's the Yellow/Green/Blue block:
Also he didn't fit an aerial adapter for the power aerial so I can't pick up hardly any stations, so I've ordered a PC5-52 too to sort that out.
Does anyone have a wiring schematic for the car online that I could reference? I'm no ICE expert but have wired stuff in plenty of HUs before so I'm happy to take this task on, so long as I have enough info upfront.
Does that all sound about right to everybody? All thoughts would be welcome.