I live in Argentina and have a 1989 190 D, gourgeos. But there this mistery no one I consulted has been able to solve.
the three knobs for the air heater on the panel (middle one distributes air flow and the other 2 control temperature for each side) heat up (real hot it burns your fingertips)
it does this 98% of the times the car runs. I found out that what does this is a couple of little red hoses that come in and out of the back of the knobs, but i canīt figure out what they are for and why it heats up so much. it happens in warm or cold wheather ans in hot or cold operating temp.

if anyone has a clue of this please give me a hint. thanks.