I'm not clear on just what you're talking about, but if it's the rear heater
of a waterboxer, you just might be in luck.

The valve that connects to the rear heater core (an inch square black plastic
piece that connects to the core with 2 bolts/nuts)  is sealed with an O ring,
available from your VW dealer for about $2.
These rings tend to flatten out with time, and could very well be the
problem.

Clamp off your hoses ( C-clamps work), put a pan under the whole heater
assembly, so as not to spill antifreeze under the bench seat, and disconnect.
(after you have gotten the new O ring.)
You will need to bleed the heater via that white plastic screw.  Be careful
of this, as they tend to break, and than you're, uh, screwed ; ).

If it is any joint within the two heater pieces (as opposed to between the
pieces) then you will have to replace them, I think.  If you can find anybody
who will repair, and guarantee these parts, please post.

If antifreeze has spilled under the seat, you will have to remove the entire
assembly to get rid of that waffle smell.  This took me about an hour and a
half, start to finish.  I had to cut some replacement masonite as an
underlayment for the seat, as the old was thoroughly soaked with 'bahn juice.

Also.  Dispose of that antifreeze correctly.  Kids and animals will drink it
if given half a chance, and die a horrible death.