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Broad question... there are so many things that could cause this... I'd test the actuator, I'd test the relay and then I'd also lubricate the pivot bushings and all pivots on the door linkage. If you go to my tech page there is a ton of info on how to test the actuator and other parts including the relay. The repairs link is listed below.
Just enter your year... then search things like Wiper, or Actuator... simple on word searches work best.