Many times the "fresh-air" door doesn't shut due to vacuum leaks in the A/C system. That will cause the warmer air to force its way into the car at higher speeds. You can see this door above the blower motor by removing the glove box liner. The door should be completely closed in the OFF position & completely open in the EC position. IF you don't see this change the vacuum elements are leaking.
THIS is only for the W126 chassis!!
MERCEDES Benz Master Guild Technician (6 TIMES)
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44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)