Ken Wilford has an article on this problem on his website (van-again.com).  He attributes it to a bad valve or a bad cable.  Either is an inexpensive fix that you can deal with temporarily (Drop the spare and close the valve by hand) or permanently (Replace the bad parts).

Chuck
'85 Wolfsburg Westy -- with temp heater fix

In a message dated Mon, 22 Jan 2001  2:40:33 PM Eastern Standard Time, Stuart MacMillan <macmillan@HOME.COM> writes:

<< Either the in-line heater  valve is bad or has become disconnected from
the dash lever wire.  You can see it if you remove the spare tire and
look up.  If the wire from the dash lever actuates the valve, then it is
not closing fully, which is a fairly common problem as the plastic ages
and shrinks.

Cheap part from the list suppliers, under $6.  Postage costs as much.
--
Stuart MacMillan
Seattle

'84 Vanagon Westfalia w/2.1
'65 MGB (Daily driver since 1969)
'74 MGB GT (Restoring sloooowly)

Assisting on Restoration (and spending OPM):
'72 MGB GT (Daughter's)
'64 MGB (Son's)

Stripped and gone but their parts live on:
'68 MGB, '73 MGB, '67 MGB GT
 >>