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Ok so here is the wiring diagram, connector view for the rear window, and rear window switch bench testing.
You should first check both the highlighted fuses.
Disconnect the rear window motor switch C351, turn key to on, test if there is continuity to the positive battery post at pins 2 and 7, test if there is continuity to the negative battery post at pins 5 and 8.
You can use the wiring diagram and the connector view to test if the switch(red arrow) supplies a ground to the correct pins of the motor connector(blue arrow) when the switch is used through its positions.
Next remove the rear window switch and bench test it using the directions of the last picture, also test if there is continuity to the negative battery post at pins 6 & 8.