Short of an RV or Roach Coach,Vehicles were/are Not Made to Have Fire in them!Buy a Bunch of Chemical Heat packs,They are Cheap,Effective,& Safe!Put em' into a .30 cal Ammo can,throw a few Clean Rags on top,to keep them from sliding around,& Short of their Expiration Dates,of which you can write on top of the can,You will have No Maintenance issue's!