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Most aftermarket places ONLY sell the 82-91 style regulators and motors as a set. If yours is a 77-81 regulator and your trying to put a 82-91 on it, they won't work, and vice versa...That is why I asked if you could post a picture of what you had. You need the same style motor that came off your regulator...
That same style motor, 12497971, is $5 cheaper at Rock Auto than Amazon.