unfortunately the seats in the z3 don't have replacement parts for the seat guts... sounds like the transmission between the actuator motor and the frame is stripped out. all that's available from bmw is the entire seat rail/frame.
you might be able to dissect yours and a used one and come up with enough parts to make one of them work, but I've never had my hands on one that deeply.
wish I had better news, but there you go.
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