Stereo rear amp shorted? - 87 5KTQ

[M Connors <mconnor@frontiernet.net>]

    Can anyone offer any pointers to troubleshooting the rear amplifier
in a 1987 5KTQ?  I have had a ongoing problem with no audio coming out
of the rear speakers with my stock Bosch radio. This has prompted me to
upgrade the stereo in the car in an attempt to fix my rear speaker
problem.  After some diagnosing this evening I now believe that there is
a "short" in the rear amp that is causing my problem.  The short theory
was confirmed when I wired in my new stereo.  I have found that if the
rear speakers are connected to the new stereo; all 4 speakers go dead. I
can not obtain any audio. (even with volume moderately pegged).  When
the rear speakers are removed the stereo works great. I was wondering if
anyone out there had any thoughts/suggestions/similar experiences? (BTW,
when I power the old or new stereo up I can hear the rear amp relay
click in the trunk.)   Was also wondering if amp failure is another
common Audi "trademark" such as (let's see now) seized door handles,
broken key locks, broken exhaust manifold studs  jumping speedo's,
radiator neck breakage  ... (Ahh, the rewards of Audi ownership).

Thanks in advance.
Mark