You do realize brakes are supposed to get hot, very hot, that's how they stop the car by converting movement into heat...that being said you have to remember the rear brakes are smaller than the fronts, have a poorer cooling system, and have more rotating mass (the larger rear tires as compared to the fronts), which all translate into higher temps

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2006 BMW X5 4.8is Lemans Blue/Beige/Dark Maple
-Nav, Comfort Seats, Heated Steering Wheel,
Rear Airbags, AUX Input
2009 BMW M3 Space Grey/Black/Carbon
-all options