To get the Grav-plates for the car, don't allow your car to be stolen in New Reno before you beat the game. (Never leave it on Virgin Street) Wait until after you've defeated Horrigan, then go back and allow it to be stolen. Get the first upgrade from T-ray and he should offer you the Grav-plates if you talk to him a second time. (At this moment, I'm not sure if there's a skill requirement, but I don't think there is)