I like the idea of a 30-30 lever action, but if that tube gets dented by something in the trunk, behind the seat, whatever, it could cause some loading trouble. Also you'd have to keep the tube loaded, which may not be a good idea legally.
A pistol caliber semi-auto like a Kel-tec, Marlin, Ruger or something similar is it's very short and handy. You can keep the mags loaded, but not in the rifle so the gun isn't legally loaded. If you carry a 9mm, pistol then you have more ammo on you. Even better if the guns use the same mags.
I'd go with ghost sights instead of a red-dot or optics in a car gun though. You want something that can put up with a lot of variations in temperature, get knocked around some and still come up ready for work.
An SKS with Mojo peep sights is probably a good truck/car rifle, certainly cheap, tough and reliable. You'd have to keep the loaded stripper clips in a pouch or 7.62x39 rounds will be rolling all over your floor boards. Fumbling with the strippers might not be as easy as slapping in a magazine either.