I have a 91 300E. The air blows out the defrost and center vents at all times. It doesn't matter if I press the bi-level button or vent only button. If fine until at night the frost starts building up on the windshield.

Where do I need to start?
pods under the dash?
or do I have a bad Climate control panel?

I have a 97' E300D with a somewhat similar problem.
My A/C blows cold out the right side of the dash but hot on the driver's side.
I've been told this is a vacume door issue but haven't gotten into it yet. Also was told it could be the "sunsensor" or pressure sensor. Ultimately we may have a similar root cause. Any thoughts?

The problem with the '91 is most likely the defrost vent vacuum pod. You are lucky, this one can be reached without removing the dash.

The problem with the '97 is most likely a low refrigerant charge.

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2012 E350
2006 Callaway SC560

On the '91 W124, the defrost pod is probably torn and/or leaking. The default position is defrost ON, and engine vacuum pulls it to closed position (OFF), but it can't/won't be pulled if it is leaking. I have never replaced one on a W124, but on my W126 it is readily accessible and removable if you take out the glove box liner (I just did this the other day). You can rig the defrost flap closed until your new defrost pod arrives (that's what I did). I imagine all of this is probably similar for the W124.

On the '97 W210, brewtoo is correct. Check the charge first. If the charge checks out OK, check the mono- or duo-valve.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280