Why must the cars be stealable in first place? Well if they are stealable then why can't we have a chance to ask merc sit in that car and shoot anyone who tries to touch it. There's no way that anyone manages somehow sneak in the car, hotwire the engine and drive away while this merc is sitting inside the car and preventing everyone trying it.
And that's not it, if this merc was shot then we should also be able to tie the car in a pole with heavy chain. Or attach a tire lock. A strong driver could even remove the tires and carry them with him while he's not around his car.
To make it even more secure why don't players drive their cars somewhere close to the city, camoflague it and walk to the town?