If you're cheap, a Rabbit front heater core can be made to serve as a Vanagon
rear heater core.  The last one I got was brass - should last forever.  You
lose the shut off valve in this deal but you can get an inline shutoff valve
from a Rabbit that works nicely.  I turn my rear heater on once a year and
off once a year, so lifting the seat and turning the valve by hand is not a
big deal.  By the way, you have to trim the plastic heater cover to make the
new core fit.  Not pretty, but it works.

Tom
'71, '78 and '85 Westy, and '91 Carat