crash sensor 1999 s3 8l

im guessing you have an airbag light on? there is a few crash sensors around the car sides front etc, best get a diagnostics device, plug it and it will show the fault code and location of sensor, i got one off ebay for about 15 quid and works perfectly.
 
if poss get it scanned for a fault code, i believe there are a couple, g200, g202
 
yeah, you will have some in the doors, on the front panel, even in the seats meaning a wire leading under the seat may have been damaged. i am a panel beater by trade and have seen airbag lights come from dodgy seat airbags many times.