Vehicles don’t act like enclosed spaces due to missing parts. While this can be interesting and blown off as “zombies reaching through an open car door window” it makes absolutely no sense for that to be the case all the time. Beetles have it especially bad, missing an entire board that exposes the interior to weather, and being able to get attacked from the rear wheel area; the school bus is the second worse, missing two whole windows on its back half (those that are over the wheels) This vehicles.json alteration stops that crap from happening.
It also alters the large trailer into a more useful space with roofing, aisles, and a few extra boards over the wheels to fully enclose the interior space.
Due to the HUGE file size of the vehicles.json file for an all-text file (80kb!) and the multitude of changes within (which inflated it a bit more) I cannot copy/paste the text directly into a spoiler (exceeds 20k characters; multiple times) so simply follow the pastebin link to view all the altered text:
http://pastebin.com/YdmLNdsm
select ALL of it IN THE LOWER WINDOW (scroll to the bottom of the webpage) then copy/paste to your vehicles.json file; open the vehicles.json, select all the text there, press delete, then press paste. Be aware that if you’ve already made any alterations to vehicles.json, those changes you’ve made will not exist if you copy/paste everything blankly. The new vehicles probably won’t appear until you enter a new map, or start a new character (all existing vehicles remain the same) and at worst, won’t let you load your save. (replace the vehicles.json with the original for your version of cataclysm)
This is also based on the latest nightly update (14th Oct, 2013) so if you’re using another version of cataclysm (ex: the stable release, or earlier) it might not work properly and throw up errors; it should NOT throw up errors, I have play tested these changes, and it is fully functional on the latest nightly update (14th Oct, 2013)
Hopefully, this is simple enough to implement in the official version.