[Goth]

I had a similar problem with my 84 Carrera. The wiper switch has copper arms with the contacts on the ends. These arms get bent over time and need a little tweak. Look at how the wiper switch operates. You will see that when you move the lever, it makes some contacts close while others may open. The contacts are pretty easy to spot and adjust, as they are like the ones you would find in many relays (looks like point type on flexible copper arm). You can bend the copper arm so the contact is in the correct position.

Hope This Helps..

EDIT: I may have cleaned the contacts as well (as Brian suggested).