You've already covered a lot of the obvious places to check. Somehow either the battery is not supplying the neccessary voltage or the ignition switch is not allowing voltage to get past it. Another possibility is a broken wire which you can find by by working backwards from the battery using a wiring diagram.
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.