Rear speaker "hiss" with aftermarket head unit

[AudiJeep@aol.com]

I have a Sony CD player in my '92 100 (35x4, 14 watts RMS). I have it hooked
up correctly (at least to my Bentley's wiring diagram). Here's the problem.
With the CD playing, there is quite a bit of hiss from my rear speakers (facto
ry Nokia's, not Bose). It also does this to a lesser extent with the radio.
The front speakers have absolutely no hiss whatsoever with either the CD or
radio. Local audio guy said to get a noise filter, but that is only for
alternator interference, right? This hiss is there when the car is on or off,
so that rules out the alternator. I know the rear speakers are amplified, and
the fronts are not. Could this be causing this? I appreciate any help. TIA.

Craig
'92 100S (67k)