They are different sensors. The ABS one goes in through the backplate. Its item 4 here:
http://www.realoem.com/bmw/showparts...45&hg=31&fg=10
The break wear one sits in the brake pad. Its on the end of the yellow sleeved wire that goes into the caliper
Havent got Bentleys handy but if you check the resistances of the ABS sensors and compare them you will probably find one thats way out. The tip can also attract crap leading to a false reading but you have to get your hands dirty to do that :-)