I don't recall an airbag mod, but I assume it's something about not taking damage in collisions? I also recall a 'seatbelts mod' for GTA4 that made it so you wouldn't go flying out the windshield in a collision. That could be doable, probably a constant somewhere that determines the force to go flying, but of course that would make a challenge unobtainable. Still, could be doable.. could even do the opposite to make it more likely for people (and yourself) to go flying through windshields at the slightest stop.