SOLVED: 1990 300SL Instrument Cluster Problem
Well, thanks to gillybenztech the problem with the instrument cluster is solved. I replaced the instrument cluster circuit board which is actually everything but the gauges and the front lense. This took care of the mystery of the gauges not working. Please see prior posts on this topic for the tests Gilly provided which lead to the correct diagnosis. The part cost me ~$412. I still have some other electrical gremlins to work through regarding the turn signals, but I wanted to post how the instrument cluster problem was solved.
Thanks to all,
1994 E320 Coupe (120k)
1990 300SL (BBS,RENNtech adds)
1988 260E Sedan (180k)