> 1 - original wiring uses a common negative speaker lead.  Most real
> stereos need a dedicated lead for each channel.
>
> 2- I have to look, but I think the rear stock speakers may be 4 ohms.
> At lot of stereos will not happily dry 4 ohms, but are meant for 8

I agree that it sounds like either a ground or impedance problem.  Other
possibilities I can think of are that the positive connection between a
speaker and the indash is arcing (sp?) to ground somewhere along the way. Or
that there is damage to one or both of the rear speaker voice coils.  I'd
try disconnecting one speaker at a time from the back of the indash and see
if you can isolate the problem to a single channel.

- Ron Salmon
  The Bus Depot, Inc.
  http://www.busdepot.com
  (215) 234-VWVW