About cars, road injuries are responsible for about 1.2 million deaths per year, they are extremely dangerous death machines, so I think it is reasonable to say that a car can’t be unarmed, though I agree that it would stretch it. By that definition, a large wrench can be a weapon, so I am hesitant to just call anything that can be used as a tool for violence as a weapon, because almost anything can be… I have a pretty heavy keyboard which could be used as a weapon if I really wanted to.
If you consider a weapon as an instrument that increases the attack potency or range of the wielder, a car is certainly can be used as a weapon… We even require people to have a license because of how dangerous they are, just like weapons.
If you consider only something that was designed for the purpose of increasing the attack potency or range of the wielder, then a car isn’t one. It all boils down to how you define a weapon.
And about the FTL thing, assuming it is possible, I can still think of a couple of ways any relativistic/FTL ship can be used as a weapon even without using it’s kinetic energy for impacts.
Blue shift of electromagnetic radiation. If you are getting closer to the target at either relativistic or FTL speeds and you release electromagnetic radiation (not necessarily visible light, even a powerful radio, which I’d imagine all interstellar ships would need in order to communicate over enormous distances), or even just a regular thruster… the blue shift would turn it into extremely lethal, short wavelength, somewhere in the deep X ray.
If the FTL system works by stretching and compressing spacetime around it to travel distances with some kind of field… It would be theoretically possible to asymmetrically stretch space in a way that would wreck a target’s structural integrity, and depending how aggressive you can take it, go full blown spaghettification like black holes do.