like someone else said - the drains are the biggest problem.   the felt seal
you see around the perimeter is only supposed to keep the majority of
rainwater out.  i've owned 3 sunroof vanagons, and one sunroof bus.  i have
never had a problem of leakage, and a few of the vans sunroof seals were on
their way out.  

when the weather is nice (not 15 degrees out..you open the sunroof when it's
as cold as it is now, you'll snap the friggin cable...DONT try it)  open the
sunroof, and take a spray hose and blast out the top drain holes.   water
should flow freely out the bottom drain holes behind the front wheels.

chris
'84 wolfsburg vanagon w/ super sunroof