I have this this same problem (leaks from rear heater).  I bought a new core for $100 (not original) and found the switch (or valve) under the seat might leak also.  But I didn't change it that time.  Few weeks later it still leaks.  I explore the switch to the parts and found the sealing (or rather hole rubber plate) of the shaft has worn out.  What should I do?..buy a new part (expensive)..put a common pipe valve (made of brass) instead (mucho work)..

I decided to make a same size/thickness blind rubber plate => now the switch is always on..but it is also really cold here in Finland and you can adjust amount of heat by adjusting the speed of the fan.  

Juha Koivisto
88 Vanagon (Caravelle) 1,6TD (old school bus with 9 seats) 
TURKU, FINLAND

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