I'm having the same problem with my new 94 RMW. Just bought it down in Carmel, CA a few days ago and drove it up to Seattle. When I started driving it, the gauge was working fine, and immediately after I bought it, I went to a local shop and had a new starter put in it. When I was on the road again heading to Seattle, that's when I noticed that the gauge had stopped working. Unfortunately, I couldn't turn back to the shop to have them re-connect the wire if it came loose. I read that the sensor for the temp gauge is adjacent to the starter, but without a jack to lift the car up, I can't squeeze underneath to investigate!
Did you get it working again? If so, what was your remedy?