the easist car to use is seige tank because it can seige to exit
have a location center the unit you want to enter
when a tank(car) goes to the location, give the tank to the player, move the unit (or remove) and center the same location on that unit again. if that player seiges, then have a wait trigger so the player can get out of the unit (order it to move somewhere so it will always get out works too), remove the unit move the original unit back and have the tank created again. i created a tutorial for this but it got lost, perhaps someone has it
