Sean,
    I had the same problem.  I removed the switch and used contact cleaner
to clean the switch.  Squirt liberal amounts of cleaner into the switch,
it's a tight squeeze.  Repeatedly cycle the switch with it full of contact
cleaner.  Use compressed air or air dry the switch.  When dry test with a
multimeter or reinstall and test.  It took two attempts to get my switch to
work.  It has been working for over a year now with no problems.  I am using
50w bulbs in the back up lights.  I can actualy see behind the van at night.
     Hope this helps.
Colin A Moran

K&M Ranch