I just finished installing new kit as below in my 96 Audi S6.
I am running this with my exisitng Sony head unit with 4 channels out , 2 Front and 2 rear
Amp JBL GTO 540
Front Components 5.25 JBL P560C
Rear 6x9 JBL P963
Wiring kit FLI Audio AK10 10AWG.
New speaker cable.
I wired the +ve to the battery (under the rear seat) and the -ve to the chassis mount of the ABS unit (under the rear seat)
With the ignition off the sound is great but with the engine running I get a speed related whining noise through all the speakers.
Did a bit of checking and will try the following.
Get a better earth point
Reroute the power cables away from the RCA signal cables.
If this doesn't work can someone suggest a suitable noise suppressor to fit in line with the positive cable to get rid of this interference