It would 100x easier just to run the aftermarket heaters on their own wiring and circuits and then just use the Factory X5 Heated Seat button and get it to work with the new wiring.

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2002 BMW X5 4.6is soon to be LSX swapped. Silver on Imola

2003 BMW X5 3.0i 5 Speed Manual, Sport Pkg, 190,xxx miles and counting. Black/ Black 20" 4.6is Wheels, OEM Stainless Steps, ZHP Shift Knob