I don’t have a vehicle rifle; Although I'd like to have one. What I would expect to have in a rifle such as that would be a compact, mid powered, durable, reliable, and easily reloadable rifle.
My First thought would be something like a folding stock AK-47. It's accurate enough, has good all around "firepower" when it comes to the power of the 7.62x39 and the ability to have extra and easily changeable magazines on hand, it should stand up to the environmental changes inside your vehicle and always run without a hitch, it's not so nice that you would be afraid of banging it up a bit, and with an under folding stock, you might be able to cram it under or behind a seat without much trouble.
There might be a way that you could lock it to your vehicle with a keyed cable bike lock. It wont be Ready to go that way, but one would hope you would already have a hand gun handy on your person for an IMEDIATE threat, then if you needed to, or it's in your best interest, you could take your car keys and unlock the rifle from the cable, load the magazine, chamber a round and go from there.
Obviously the best course of action would be avoiding or escaping trouble in the first place. But I suppose there might be an occasional reason to have a good rifle in your vehicle for defensive reasons, no matter where in our great country you live. Take one from the Boy Scouts and "Always be Prepared"
However, in my opinion, always being prepared also means securing the rifle, or at least hiding it very well incase your vehicle is broken into.
Like mentioned above, the Kel Tec Sub-2000 could also be a great option, with it folding in half it could easily be stored in some of those hidden compartments many vehicles have that are meant for Jacks or Jumper Cables... but then what do you do with your Jack and Jumper cables ???
There is nothing wrong with a good bolt or lever gun either. They would be better to have than nothing, and in some cases may have an advantage over other types of rifles.
In the end, it’s up to you to decide, based on where you live, and what uses you have in mind for the rifle, what to get. And how secure/accessible you want it to be inside your vehicle.
Dont forget... if you think you want a defensive rifle in your vehicle for protection of your self/friends/and family; Consider what else is necessary for your body to survive. Food, Water, Warmth, a knife, Communication.... Easily accessible self defense like a handgun. Dont forget a flashlight. Extra fluids for your vehicle??? so on and so on....