Well, this is what worked for me in my 1981 300tdt.
I am assuming that the vacuum systems for the heat are very similar in other cars...but I may be wrong.
I removed both drivers and passenger side below-dash panels.
Then I turned on the heat to high. Noticed that the vent flaps were all the way up...as if the defrost were on.
I pushed the button next to defrost so that I could at least see what it did to the vents if anything and got nothing. I stuck my hand up there and manually assisted the vents down. Only took a little push to the back of it and down they s-l-o-w-l-y came.
But it DID come down and I had heat pouring out of there. (hold on, not done yet!)
THEN, I pushed the defrost button and they went back up...slow.
Good, I thought, and pushed the button next to the defrost again, and NOTHING! bummer.
Dehind the center console just behind the footwell vent opening on the drivers side is the vacuum pump (I am assuming that is what it is) for those vents.
I wiggled the vac. line at the rubber connector and BOOM! down the vents came, much quicker than the first time. I repeated the defrost, vents go up, but then again they stuck and would not come down on their own. I wiggled the line again and they went back down. I WAS GETTING CLOSER!!!
SO, by process of elimination I replaced the rubber connection to the pump on the drivers side with a 2" piece of rubber tubing...
(drum roll please!)
NOW I HAVE FOOTWELL HEAT!!!!!!!
Consistant and QUICK. It appears that there was some kind of restriction in the original rubber connection, that may or may-not have been there for a reason, as I could not blow thru the original tube. No adverse effects can be seen so far by replacing it with a hollow one. Only good things like FOOTWELL HEAT!
(Even turns off and on automatically based on temp of air coming out!)
Sorry for the long post, but I am very excited to have that problem fixed.
Now onto the locks and hard shifting...
