<e30> Replacing Stereo Head Unit - More Stupid Questions

[Chris Baisley <cbaisley@xxxxxxxxxxxxx>]

The head unit in my 89 325i is making me feel really dumb.  I've installed
several car stereos in non-BMWs before, but my preliminary pokings around
have got me a bit stumped.  Any assistance is appreciated.

Question 1 - Are There Speaker Relays In the System?

I have factory japanese anti-theft BMW head unit, 5.5wpc according to the
owners manual (which also says that it shouldn't be used with an outboard
amp), and apparantly no outboard amplifier.  I can't see one in the trunk
(haven't removed liner though), and it sure doesn't SOUND like there is an
amplifier anywhere.  But, if I disconnect the power antenna lead from the
head unit, I get no sound from tape or cassette.  It seems that the 12v out
from the power antenna lead not only activates the antenna, but flips a
relay (or turns on an amp that I don't think I have!) that completes all
four speaker circuits.  I confirmed this by hooking up a tone generator to
one of the speaker circuts (between +spkr and radio chassis ground) and no
sound made it to the speaker until the power antenna wire was hooked into
the head unit.  What the heck is going on?  Is there an isolation relay for
each speaker circuit?

Question 2 - Using Chassis Ground for Speaker Circuits

My experiments and examination of the wiring lead me to conclude that the
system uses chassis ground as the negative half of each speaker circuit.
Can someone confirm therefore that I simply need to hook the negative leads
from the new head unit to ground to make the new unit work?  I was hoping
to avoid installing all new speaker wire (+ and - complete loops) until later.


- -- 
Chris Baisley
cbaisley@domain.elided (Personal)
baisley@domain.elided      (Business)

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