If I am shooting at you and see you get out and hide behind the door I'm going to start blowing holes in the driver door, bullets goes through doors. They aren't magic bullet stoppers. Because you aren't shooting at me, I might take that opportunity to advance on you and shoot you in the back as you are crawling in the floor of the truck.
If I am in the truck and you start shooting at me, I might suggest taking that opportunity to put the truck in gear and haul butt out of there, spraying dirt and gravel over the shooter to temporarily distract him. I did that once to an unarmed attacker and they dove for cover from the dirt slinging off the tires.