I've seen alot of posts having this issue, and i can't figure it out myself either.
The normal vehicles work and actually pullover that the addon pack replaced, but when i'm trying a custom one vehicle that the pack included nothing works.
The only thing that it shows that is, when i press shift that the blip shows up and nothing else.
+ when i put on my sirens the ai do not react to it at all, thinking it has to do with that too.