It sounds like a bad change over vacuum valve. Its one of those five valves behind your ACC. Thank God your vacuum element for the center vents is holding the vacuum. Otherwise its PIA to change that element. it requires you to remove the whole dash board.

Now in cold weather foot vent will not open at all until you have acheived sufficient temp during engine warmup. This way it wont blow cold air on yr foot. Only after the coolant temp has reached above a certain value, the legroom vent will open.
Now that its warmer weather, start the car, let it come to about 50C or more, press economy push button, and set temp dial to hot do not engage in hot, and see what happens, if it opens the leg room vents( in about 5-10 seconds) I think you are fine. If not
you have 2 possible senario( assuming that all vacuum elements and vacuum lines are fine)
1. A bad ACC unit.
2. One or more bad change over valve.

While you have access to all 5 valves at this point, I would test the function of each of them. Its a very simple test described by MB repair CD( I can attach PDF if you send me your e-mail). Once you have done this and found the bad valve, replace it because you will need to replace that valve( or valves) irrespective of your ACC console was good or bad. Once you have replaced them test the system again and see if all flaps open and close as per the owners manual. If so you are all set. Its only the valves and not ACC.

Also no or little or irrational behaviour of heat can be due to a bad monovalve. Its very simple to inspect and repair( about 25/- bucks for repair kit) if necessary. A second reason for delayed heat esp during idle is a non functional auxillary water pump. its funcionality can be checked very easily too. Locate the Auxillary water pump, remove a little black cap on the back side of the pump with a fine screwdriver( see my previous posts for a photo)
Now start your car press defrost button, and touch the back side of the auxillary water pump. you should feel a rotating shaft there, this tells you your pump is working fine. Now press the economy button and set the dial on heat(red), you should again feel the rotating shaft. This is good. Now press turn the dial to full cold and feel the shaft. After a few seconds(and as much as about 30-60 seconds) feel the shaft on the back end of that pump again. It should have stopped now. If it did stop Congrats your Aux water pump is fine.
If during the "shaft feel test" in defrost mode, the shaft feels not turning you have a non functinal aux water pump. Next step is to disconnect the pump, from the socket and test it with a direct supply from the battery or any other 12V DC, if you feel the pump is working(shaft can be felt turning) then the power supply from ACC to this pump is not working. To confirm this press the defrost button while engine is on, and measure the volts across the the 2 points on the female plug it should be about 12 V. If there is no voltage its an electrical problem and is mostly due to a Fried ACC or broken connection.

I know I gave you too much details but I went thru a hell to learn all this last week and I wanted to be as easy for you and other as I can be. This description applies to my 1985 300D turbo 123.

Let me know how it goes. My hunch is you have atleast one bad change over vlave, a busted or nearly busted monovalve. and a nonfunctional auxillary water pump(or may be not).

Good luck
MVK

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1985 300D Turbo 179,000miles