Fixed it. I messed up copying the coding into the vehicles.meta file for one of the vehicle models I added. Thanks for your help!
Edit: Not too sure how to mark thread as solved.
Hey guys,
I've added a few vehicle models into the game replacing the police vehicles (specifically fbi, fbi2, police, police2, police3, police4, sheriff and sheriff2). I've never had this crash before. I made sure Rage was up to date to no effect. Any help would be appreciated. Hope I've got the right thread aswell.