Have you checked the temp sender connection? I can't remember where the sender is on the '7 (the front of mine has been buried in a garage under a pile of junk for two years), I think it's on the radiator though. I Get this often on my Saab, the sender wire falls off and I get zero reading on the temp gauge.
Check the easy stuff before you go ripping the dash apart! [:)]
Garry
1976 2.0 (soon to be 3.5!)FHC
http://reds-tr7.blogspot.com/