shadyraven

HU
PCB
mini-ISO

Sorry, I just installed the changer following the instructions. It works, but I don't actually know what's what.

TwentyValveB5+S4

But when I was hardwiring the radar detector into my 90 via the sunroof wires, the brown was the positive lead and the black was the negative.

And the ground to the deck was black too... but, could be brown. Get a line tester/voltmeter!

MacSub

I'd try the chunky earth to the back of the HU first. What I did was burn a CD with silence on it, play it on the changer with the engine running, jack the volume on the HU right up and then with the earth lead firmy bolted to the back of the HU, tried the other end on various chassis points on the car bodywork behind the glovebox. In this way you can really hear the difference the additional earth is making and get the best point for noise reduction.
Use a 4 guage earth strap and keep it as short as possible. I ended up using 2 attached to different points on the car. You may also want to piggyback a small earth lead onto the screen wire for the BUS cable (detailed on the HU as "COM" next to the CLOCK pin) and attach the other end to the HU chassis. This is really fiddly which is why I just soldered my new lead directly to the PCB.
Also ensure the changer is really well earthed in the boot and add an additional earth lead there as well. The BUS cable is of pretty low grade quality and the screen (the earthed shield that protects the low level audio) is pretty feeble so you need to ensure it is well earthed at each end.
This should be an exact science but in my experience you just have to solve it by trial and error. I actually found that positioning an additional earth strap in the wrong place actually makes the interference worse, which doesn't make any sense, so you just have to try various things and use whatever works best.

shadyraven

MacSub, thanks for the thorough write-up. Much appreicated!