At the moment I keep a SOG POWERLOCK and a Cold Steel Folding Bushman in the car.
A multi-tool is handy and I really appreciate them come flat tire time.
I used to keep a large fixed blade in my car, often a kukri of some sort, in addition to a multi-tool, since I always have an EDC or two on me. But, with my current car space is at a premium (can't even carry a rifle like I used to).
A "car knife" to me is an emergency tool, so they see little actual use.