It is the spoiler that goes across the upper portion of the rear deck, ala Ferrari Boxer.
Look at a GT5 or GT5S to see one as they came with them from the factory.
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.