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Very carefully, go and move it using your fingers. these things are old, and between dirt, water, dew, and wear & tear they start to show their wrinkles.
If the mirror on the passenger side moves OK then the wiring/switches are probably OK.
Pull the mirror out of the housing and check the connector for corrosion/connection. As I recall there is another mirror connector inside the door. The motor might be bad or the little screw thing (threaded shaft) that moves the mirror might have come out of its socket. While it's apart, be sure to tighten up the screws that hold the motor mounting bracket into the mirror housing and the screws that hold the motor assembly to the motor mounting bracket. Seems like they ALL get loose over time.
Here's a thread with a bunch of pictures that will probably be useful: