I had trouble with my '77 450slc heater blowing hot air (more of which I don't need in S. Texas) down the driver's side door and around my feet. The mechanic (factury trained who works for the local dealer but on the side in the evenings at half the hourly rate) said the problem is that one of the vacuum pot that controls the hot water flow is bad. Not only does it cause the water control valve not to close but the leaking vacuum causes the engine to run a bit rough and deprives vacuum from other pots in the system. The pot is only about $20 and a bit of labor. I am sure that the entire heating system is crawling with these pots-anyone of which could be leaking.