Problem adding lightbars to vehicles -Modding related

I've tried a few times on a few different vehicles to add the new game lightbars but no matter what I do they just don't work. It will show up fine but I can't get it to attach, it'll either just drop off or flop around. I use the correct slot name but other than that I can't see what else needs to be changed. Any help would be much appreciated.
[edit]
I should add that the vehicles already have lightbar slots, I'm just trying to add the new ones.

 

I guess I didn't explain myself well enough. I'm trying to add the lightbar code to a mod.