An E36 blower that only works on the highest setting (4) has a failed final-stage resistor (the FSU). On full speed the motor is fed directly and bypasses the regulating stage, so if only high works you've already proven the motor, wiring, power and ground are all fine — the fault is the resistor.
It mounts on the heater/blower housing with heat-sink fins in the air stream, reached from the cabin footwell (driver's side on left-hand-drive ca
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