I'd dissect the old one. See if anything is obvious. Then I'd try a replacement. Nowadays electronic failures are almost expected. If that fails its time to start doing checks to make sure the wires are going where you think they are going.
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.