If your unit has continuously-variable blower speed,  the "power
stage" and is a heat sink for one or more transistors that supply the
current to the fan motor.  The factory manual lsits either DTC codes
(connector B1) or substitution for troubleshooting this part.  

The schematic also shows a manual system, where there there are
several fixed blower speeds. This uses a resistor pack
Pin 4 (green wire) is the common, and three resistors go from there to
pins 1,2,3  (black, black/white, brown)

They don't show the resistor values, but if one was bad, it should
still work on some speeds.
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