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re: E28 stereo wiring

Jonathan states his problem is power being cut off to his stereo when 
he steps on the brakes.  Been there.  Had that same problem with my 
aftermarket stereo.  What brand did you buy?  Not that it matters, 
just curious.  I have a pioneer.

Anyway, the fix is simple enuf.  What I did (if I remember correctly) 
was stick my voltmeter into the ignition controled wire, then stepped 
on the brakes.  I wanted to see the voltage drop occur.  Sure enuf it 
did.  I bet you also hear a periodic pop from the speakers when you 
apply the brakes, that is, if the stereo doesn't turn off first.

After you check the ignition line, find a hot lead (ie one that is 
always hot).  Check it also for the voltage drop when brakes are 
applied.  What you'll probably find is that it too drops, but not 
nearly as much.  By the way, I did the voltage drop tests with the car 
running.  

Now you have two solutions....

1.  Connect the radio to the continuously hot lead.  This will let you 
turn on the radio without the ignition on.  You may or may not be able 
to hear anything depending on whether and how you have an external amp 
hooked up.  I chose not to go this route.

Rather... I did this...

2.  Buy a simple relay at Radio Shack.  Look at the wiring diagram on 
the box and connect the hot lead to the incoming line, the stereo 
power line to the outgoing line.  Now you can use the ignition lead as 
the trigger to trip the relay and power the stereo like before, only 
the brake related problems should vanish.

Hope this helps some.  I suffered thru this same problem for a couple 
of years before some smart guy at Circuit City suggested the relay 
trick.

As always, YMMV
Good luck
Jeff Wilcox
88 535i 93K