By any means at your disposal,get the codes,these systems are too difficult to diagnosis without them. Myself, the most frequent airbag problem with the Z4 was the passenger seat occupancy detector. Not so hard to replace, the guys even like doing some clean work occasionaly. Has a door panel ever been off?(there are a half a dozen reasons at minimum to take a Z4’s door panel off) perhaps a side airbag is just disconnected. Still cost you a $100.00