Wiring new stereo in 10spkr 5kCS???

[Matt Bartholomew <bartmolo@mailbox.syr.edu>]

I'm trying to replace the original Blaupunkt unit in my '88 5000 CS
quattro with a SONY deck.  The problem is that the rear speakers no longer
work with the SONY unit hooked in.  The speakers work fine with the
Blaupunkt, and there's nothing wrong with the SONY.

If someone out there has replaced the original deck in their 10 speaker
system, could ya clue me in?

I have three wires left unattached after inserting the SONY (open to
Bentley page 97.600 if you're following along).  First is the 0.5mm GY/BL
wire which goes to an "Illumination Lites Booster" which is not further
indicated in the book. Second is the 0.5mm R/Y (just Y in Bentley) which
goes to the "Seat Belt Warning System". Third is the 1.0mm R/Y (just Y in
Bentley).  

The third wire goes to the rear booster, and is where I assume the problem
is.  I tried grounding this wire (as well as the other two) with no
effect.  

So, I have to believe one of you fine citizens has replaced the original
Blaupunkt in the 10 speaker system, and would appreciate your input on how
you overcame this wiring glitch.  Thank you!

Matt
'88 5000 CS quattro
128k miles