Need your help for a W140 excorcism

[A. Rosich]

Have done several searches for this problem, but I can only find related answers for W123s, W124s and some W126s, nothing in specific for a W140.

My 1998 S 500 L, when set in the full AUTO air conditioning (A/C) mode, directs some cold air through ONLY the left (driver's) side of the defrost vent at the windshield (besides the normal flow through the dash vents), making an annoying condensation problem on humid days outside the windshield.

If I select the MANUAL mode for the left side, the problem stops! Although this is unconvinient and not normal.

Any suggestions on what could be wrong or what to look for to cure this I would really appreciate it, thanks in advance!