That is a speed control device with about 3 resistors connected in series. Different switch positions put one or more of the resistors in series with the motor, resulting in different blower speeds. You've got a 12V motor but it only runs at full speed in the "high fan" position. If it's like most of the GM blower motors of that vintage, there's a relay that bypasses the resistors for high fan operation.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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