Audio Systems: car audio buzz, logic signals, toyota avalon

Question
I installed a Sony HU in a 1998 Toyota Avalon, seven speaker system, 170 watt factory amp. To use all the speakers,I set the front speakers to the new HU and sent the rca outs (which can be set up for use with a separate amp)to the Factory amp. Everything worked pretty good, but if the dash illumination is turned up, the is a major buzz, but only from the back speakers. There were illumination wires running to the OEM player, which I left unconnected,but are still in the area since the original plug is still there.  

Answer
You sound pretty handy so try this in order to find out where the buzz is coming from.

ONE:
- Remove the remote turn-on wire from the factory amp.
- Run a separate wire to turn the factory amp on.  Or you can just jumper from the constant 12v line over to to the turn-on connection.  

This will tell you if the buzz is coming in from the remote line (which is from the HU on/off - which has logic signals on it which may be a source of the buzzing).

If that does not point to the problem, then try this.

TWO:
- Remove the RCA lines to the factory amp.  The buzzing should go away.
- Connect only one channel of the RCA lines.  Try it in the left, then the right.

This will tell you if the buzzing is coming through the audio out from the HU.  If it is then the HU is the problem and not the factory rear amp.  

Check, then, the wiring to the HU to see if you have it mixed up with the wiring for the car lighting.  Try to get a direct connection from battery to the HU power line.

Hope this helps with a solution.