From the wiring diagram I have, it looks like it goes to a light (so you can find the lighter at night I guess).
Here are a couple of diagrams from PanteraPlace that show how it's hooked up...
Illumination Control
Cigar Lighter Wiring
It would appear that the orange wire leaves the cigar lighter lamp, and is then grounded. I can't say that I've ever seen the light they're talking about, but then again, my Pantera came without the lighter hooked up and the lamp may have fallen victim to the interior re-do by the previous owner.
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.