Reverse switch

I have a problem with my mk1 golf gti cab.
I have no reverse lights.
I have located the unit on the gear box and have taken it off.
There are 4 wires going into the top and 3 little plastic plungers inside.
Can anybody tell me which plungers do what and how i can overcome this problem, either by adjusting the wires or managing to get hold of a replacement unit.
Thanks speno

Look on Ebay there's a brand new one on at the morment