I had a local reputable upholstery shop install an aftermarket seat heater in my 528. They removed the seat, took it apart, installed the heat pads in the base and back, installed a factory BMW switch and repaired a torn seam in my seat- $275. This shop is where both my BMW and Audi dealers send cars for seat and upholstery repairs. The seat heater works exactly like the factory heater in my X5.

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