I had a sqeaky serpentine belt on my '92 300e recently. I tried tightening the belt and the squeak got worse. It turned out that the tensioner was shot.

The tensioner is a weak link on the M103 and fails fairly often without warning. The only way I know of telling if the tensioner is bad is that it cannot be adjusted (either the pointer arrow will be off or you will run out of threads on the adjuster rod).

Cost of a tensioner is about $150 and it took me about 5 hours to change.