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....
After driving around with the switch fixed by this method I've noticed the backup lights will come on in 5th gear when I first hit that gear or hold the shifter far into 5th.