I installed a Parrot CK3100 LCD Phone system into my 2008 Audi A4 Avant using an ISO lead suitable for my Concert radio.

Due to the switch live being unusable, as it's controlled by the radio, I wired in a switch live from the fuse box using fuse number five, which is labelled for the telephone and heated seats.
If you use the switch live from the radio,(Brown Wire), the parrot will turn on/off when ever you turn the radio on/off.
The ISO lead supplies live and earth feed to the parrot system through the connectors which are fused, so I didn't have to cut into the loom there.

The parrot connects to my phone via Bluetooth automatically when I turn on the ignition and everything worked perfectly, speakers muted etc.

The first time I turned the headlights on a day after install I noticed a slight buzzing coming from the rear tweeters while the radio was on, the speakers at top front corner of rear passenger doors, when i turned the lights off it went away, lights on and it came back, turned on the interior light and it stopped again, interior light off and the buzzing came back, everything was fine with the headlights off.

The following day I turned on the headlights and there was no buzzing, then I noticed I now have no sound coming from the rear speakers at all, main or tweeters.
Fade the radio to rear and nothing, sound only from front middle, front door and front door tweeters.

I know the car rear speakers are controlled by the amp behind the off/side tail light and could be a problem but I'm sure the rears worked before I turned on the headlights...

Personally I thinking of giving the parrot system a direct earth to the chassis as it seems like an earthing problem.

If anyone could help I would be very grateful.