I am sure that the sensor detects a drop in resistance, hence people put reisitors in place to fool the system into thinking everything is ok and no lights come on.

my e39 passenger one if playing up, when somebody sits in the seat the light comes on and after a while when nobody is in the seat, it will go off again until it is sat on again.

here is an ebay example of what you can get
http://cgi.ebay.co.uk/BMW-E46-E36-E3...item2a0d35f55b