Mbl98431

I have a 2004 A8 L with a hall sensor malfunction on the rear passenger side parking brake system. Triggers the constantly on parking brake warning light. I want to replace the right part. Are the sensors in question located in the motor unit or the caliper unit. I am able to source both parts separately rather than $1500 from the dealer. I do not know how this particular system works. Any help is greatly appreciated!!