Here:
http://www.brokevw.com/reverseswitch.html is all the pictures and some good info.
The link shows how these go bad and sorta how to fix 'em.
But what I did after having the backup lights start flickering again after putting in a new switch only 2ish years ago, is I took the switch apart, cleaned the burnt contacts as good as I could and then flipped the "button" over.
The reverse button is the farthest forward one when it's installed, and only the mk1/four wire switch is easy to disassemble, all the others are glued/welded together.
After doing this either relay the backup lights or get LED bulbs, I've never seen 'em just heard about 'em....