What may be happening in your case is the plastic gear has not totally failed and functions OK when disconnected from the assembly and allowed to turn freely. However the load and weight of actually moving the entire headlight bar assembly may be causing the inner metal hub to disengage due to small cracking rather than total gear failure.
Have you actually viewed the plastic gear in the headlight drive mechanism?
Larry
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.