I replaced the rear bench seat heater core in my 85 Vanagon with a Ford
heater core the same size and used a plumbers in-line valve instead of the
factory replacement.  Cost:  $25.00 for the core and $0.85 for the valve.
 Just go into an auto parts store and have them bring out heater cores until
youfind one the same dimension (nearly) as yours.  

Russell Gunter
85 GL
93 EV
79 SBeetle Conv