I've fixed two 'definetely dead' wiper motors buy making up an earth strap to go between a mounting bolt and one of the bolts that holds the motor gearbox cover on.
The standard motor uses a small flexible bit of steel to earth the motor past the isolating rubber mounts. The flexible steel, er, flexes and breaks, or becomes coroded preventing the motor from operating since none of the wires in the plug are earths. A quick test with a bit of wire between the metal lid of the motor gearbox and the vehicle chassis should show you if it's going to work or not.
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