dazedtraveler,
Why can't you open carry inside a vehicle in Montana on the person? This is the definition of concealed in Montana law:
45-8-315. Definition. "Concealed weapon" means any weapon mentioned in Link Removed through Link Removed and Link Removed through Link Removed that is wholly or partially covered by the clothing or wearing apparel of the person carrying or bearing the weapon, except that for purposes of Link Removed through Link Removed, concealed weapon means a handgun or a knife with a blade 4 or more inches in length that is wholly or partially covered by the clothing or wearing apparel of the person carrying or bearing the weapon.
I don't see anything in there specifying a firearm is concealed just because it is on the person's body inside a vehicle, so long as no clothing or wearing apparel is covering it.