I have been having RC90 nightmares very similar to johnclark1102. My current build RC90'd a few times, worked once then went back to RC90-ing. I was going to start my own topic until I happened on this one. I double checked no materials in "default materials" and removed collision objects other than the base CVPI (bxbugs). With this latest export, it now works!
Interestingly enough, when I turned the collision objects on for my lightbar (PFC's liberty) hadn't moved with my lightbar during assembly/positioning. This meant that the lightbar collision object was within the CVPI collision object. I didn't need to move the rambar but decided to delete its collision object anyway. I might do a quick test at some point with another car testing collision objects on/off.
RC90 seems to do with collisions as TheGTAMitch suggests....