I'm partial to the "Battery Tender", a load-sensing trickle-charger, on both the Pantera and my bikes. You probably have a minor short somewhere since you say the ground wire must also be disconnected. I'd get a good VOM and start disconnecting wires or fuses until I got one to spark when I pulled it, then begin trouble-shooting there.
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.