Get in the vehicle, lock the doors, roll up the windows, and don't speak to the person unless you're fairly sure he's not a threat. If so, crack the window and ask him what he wants.
Don't draw unless you're being attacked. Don't threaten or mention your weapon at all.
If he attempts to enter the vehicle, then it's not a bad plan to display the weapon, but at the same time you can use that time to start the engine.