That sounds like the foam hose from the dash sensor to the heater box has deteriorated. You get at it by pulling the glove box. You can buy the expensive part from a dealer, or rig up something with pipe insulation from Home Despot. Several posts on how-to.

This hose causes air from the cabin to be pulled past the dash sensor, so the sensor can get a read on cabin temp.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe