I just put a few antennas on my CVPI and now I'm getting RC20 errors. The car worked before adding them. 

 

However, in Zmodeler I just threw them into the hierarchy at the bottom, should I have placed them elsewhere? What else could be the issue?

rename them to misc_ something or attach it to whatever .. if its on the trunk attach it to boot_L0 if its on the roof attach it to Bodyshell_L0 that should solve your error I've had that happen to me a few times before

Figured it out. I did have my extra parts renamed properly.

 

The problem was the long CB antenna, it added too many polys to the car. Once I removed it, the car spawned properly.