Had a local upholstery shop (one that does sublet warranty work for the BMW and Audi dealers) install aftermarket heated seat hardware in my 528i. Just driver's side. I purchased the factory switch and they wired to that for an original look. $350 and that included heated base and back, as well as a repair to the seat back motor. Works like factory. Ask your dealer who they send their seats to for repairs.

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'04 E53, 4.4 Sport
'97 E39 528i
'86 911 Carrera, track car
'96 BMW R1100R