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Sounds like a problem with the drive mechanism. No matter what it's seat out to investigate, which is fairly easy as long as the back and forth motor works. Disconnect the battery and let it sit for 15-30 min before disconnecting any yellow connectors. Do not reconnect the battery until the connections are restored or you will get a difficult to clear airbag warning. The seatbelt anchorages have explosive pre-tensioners and the passenger seat has an occupancy sensor.