On my 1988 Mercedes 300E...

For some reason, whenever the +12V lead for the stereo headunit is connected, half of my interior lights go out. I've had numerous mechanics look at it, all to no avail. The center console lighting cuts out, some of the climate control and auxilary switches don't work... I don't get it. This is regardless of whether or not the interior light line is connected (to dim the headunit). That part works fine... It's just that whenever the wiring harness is connected to my Sony MP-70, those lights don't work. So, what I'd like to do is add another circuit devoted purely to the headunit to see if it helps. Anyone ever done this? Any suggestions? Thanks in advance.

And to get it out of the way early, it's not a grounding issue either.

-Campbell