Thanks for all the help on this problem, but we're not outta the woods yet.
My buddies 1990 heater problem is still unresolved. He has check all of the
things that you guys have mentioned, to no avail. He is looking for the
heater water valve that controls the water flow to the core. Any ideas where
this thing is located on a C4? I have a C3 ('80) and C5 so I'm no help...