Hi, I have a 1998 ML320 that won't blow warm air. My cabin filter was very dirty, and replacing it improved air flow, but still no heat. Any suggestions?

Heater operation on ML's is controlled by a blend door.

Electric motor moves flap from a signal from the control dial.

You can remove the glove box & see the blend air door.
Possibly even move the door by hand.

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Thank you for the direction. My son is driving the ML now, and he first told me the blower was making a revving noise. I wondered if the cabin filter was plugged, and it was really dirty- my bad. Do you think the backed up filter somehow caused the no-heat condition? Also, do I remove the glove box from underneath, or through the box?