I've got an aftermarket Pioneer DVD/NAV headunit in the car (AVIC-Z120BT) which has it's own bluetooth mic. It's actually taped right over the stock mic location. Absolutely no problems with it working (all the steering wheel controls work with it too!). I have, however, heard others complain about the stock setup. My guess is that maybe the mic is simply bad quality. Not sure if there is a way to fix this on a stock setup tho...