Jan,
If you have the Bosch switches like most of us, they come apart quite easily and the rocker contact inside can be cleaned. Mine was all cruddy after just two years and did exactly what you describe. The factory switches just crumble when you try to take them apart.
Mark
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.