Tomorrow I'll pull the blown rear heater out of my '84 Westy (1.9
Wasserboxer).  Does anyone have advice on which part is most likely the
leaking culprit?  I'll start my investigation there...

Also, I will need to buy those parts.  Anybody got a used one for sale that
works perfectly?  Or a suggestion on which parts place to try?  Also if it
is the core or a valve, which peripheral parts should I throw in?  Do any of
those rear heater parts go out often enough that I should get an extra to
throw in the parts bucket?

Thanks,

Matt