Hello
My 1991 2.5tdi heater switch is a rotary variable type- maybe like yours. It would only work on full. You get the inpression that the blower speed is fully variable but it isnt; there is a set of resistors in a block about 2 inches long fitted on the heater blower with two philips screws and is about level with the end of the clutch pedal, The resistors are controlled by the variable switch on the dash board. Also on the resistor block is a thermal cutout. On mine the cutout had broken so i bypassed it and the blower is now working as normal. If this is your problem and you do as i did you should realise that those wirewound resistors do get hot and are incased in plastic so this should be regarded as a temporary fix umtil you can replace the unit. The lower the fan speed the hotter the resistor pack will get as it needs to block more current. Hope I'm making sense?
Good luck.
