This thread explains it a bit, but it is on the cowling left of the cabin filters and right of the ecu box cover, kinda sitting up on top with a wire going to it. Pic in the attached thread.
2008 E93 335i, MHD, HPF IC, Injen Intake
2016 X5 M-sport 35i JB Stage 1
2013 Mercedes C63 AMG ROW airboxes, power pipe, Eurocharged V6 Flame tune 540 HP