Algae 1, This is my first post, though I have been a closet reader for 2 years at least. This one really hit home, and I just had to reply. You won't like what I'm about to tell you I'm afraid.
The exact same cold-feet thing occurred on my '91 300SE. I discovered the left footwell door hinge was broken. Here's the drill:
The two footwell doors are on either side of the bottom of the big black heater box. They are simple squares of plastic, with the hinge being vertical posts that just slip into recesses in the heater box. The upper post extends thru a hole in the box, and a plastic lever attaches to it. This lever is actuated via a complex array of metal arms that ties the left and right doors together, and then ties them to the vacuum actuater. it's really a simple implementation, MB just over-complicated it.
The vacuum actuater is the one directly behind the radio/climate control, right in the center of the dash. To confirm it's working, you have to remove the ash tray, coin tray, radio, and probably the climate control enough to see back there. As you turn the heat knob to-from full hot to full cold, you should see the vacuum actuater move the series of rods and levers going to both L and Rt doors. Of course the bad news is that the Left door is so hidden by the console and wires, tubes, etc; you can't see if it is really working even if the actuater arm is moving.
unfortunately on mine, the vertical post/hinge on the door was broken, and the actuater arm just rotated around, not opening the door. If your actuater arm moves on the left side, I'll bet you have that same broken-post issue.
I was able to buy a new door at the dealer for $20. But installation was a real bear. I removed the center console, left kick panel onthe console, lower left dash and knee bolster, and all the venting, specifically the big metal corrugated pipe under the knee bolster. These are all pretty straight forward but very time-consuming. After getting to the left door, I saw no way to install the new door, as the post/hinge needed to protrude thru the heater box hole by almost an inch, and there was no way to install it without removing the heater box and disassembling it. This would have been a few days (literally) of teardown.
After much thought, I came upon a solution: I took a drill and Dremel, and cut out a section around the post/hinge hole about 2 inches square, pulled that section of the box back, and slipped in the new door. JB Weld was then applied to replace the section of the box, and all was good.
Well, i told you you wouldn't like it. Course, it might be the vacuum, or maybe the actuater linkage just came loose. Pull the radio and peer back there and it will be clearer what to look for. If you want to proceed, I can give nitty-gritty details on removing every screw; I still see it in my sleep.