[OXO cube]

have seen somewhere people use a 2mm grub screw 
inserted into the split section of the guide post to splay the post out a little 
open the car , place the guide post into the chassis , insert the grub screw and tighten until secure , replace car body 
obviously don't screw it in too deep to prevent the guide from turning , but enough to secure it in the chassis 
that is if i understand your problem correctly 

hope this helps 

Kev