It is not a factory switch. I have seen owners mount ignition shut off switches to deter theft in that location. It may also have served an ancillary electrical device such as remote antenna, suspension lift, tail light cut-off :^)
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.