I have also had problems with the AC switch on my 86 Westfalia.  Several
years ago I had to replace the switch twice in two years.  I think the
problem is simply the fan motors are getting old and are requiring more
current to run.  This simply overpowers the less than heavy duty switch,
connectors, and wiring.  A little heat is generated in the switch,
corrosion follows, and the switch fails in a very short time.  A few
years ago I posted a description (url below) of my efforts to fix this
problem with load reduction relays (as VW calls them).  This is still
working fine.  If I were to buy a new vanagon, this would be one of the
very first things I would do, especially with the combination AC switch
selling for >$400..

If you want to measure the need for these relays, just check the voltage
available to the motor in the evaporator unit above the rear seat with
the switch set to HIGH.

http://gerry.vanagon.com/cgi-bin/wa.exe?A2=ind9804D&L=vanagon&P=R5092&D=1&O=D

Ed McLean