My guess is the sending unit. With the key on and with you being able to get at the sender, touch the end of the wire to a piece of metal and watch the gauge, if it goes to max temp the gauge and the wire is probably good, just get the correct sender for your gauge and I think it will be fixed.
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.