Sounds like your head unit's on the way out. They're notoriously troublesome. If you determine that's the case, junk it and get something aftermarket. $200 at Crutchfield will get you something far better off than the Becker.
The Bentley shop manual has a schematic. There's a lot more you'll use it for as well. This is by far the best price:
http://www.renntech.org/forums/index.php?app=spoffers&do=amazon