Thanks to all for clueing me in on my heater (didn't need it in Mexico).
This morning the van started to overheat almost as soon as I drove it and the
coolant light came on.  It was WAY low on coolant.  So I added some and
waited for it to cool down and continued.  Then I heard dripping and guzzling
from the rear heater, then an rapid perfusion of steam from--I assume--boiled
coolant filled the van.  I've clamped off the hoses, so for now it seems ok.

I have a suspicion what the problem is, but it seems easy to fix so tell me
if I'm too optimisic.  There is a whiteish plastic piece that appears to
thread like a screw down into the heater near the on/off lever.  I found the
top to this broken piece jiggling around on the floor.  Can it be as easy as
replacing this piece?

Well, my entire floor is soaked with anti-freeze--which I'll probably get to
clean up in March when it thaws off around here.

Thanks all,

Doug Demarest
Minneapolis, MN
86 Vanagon