Bleh was the only one who could do that I have yet to see him in retail.
BJWay had made a tie fighter that shot out lasers for AiO.
I don't really see why you need to mount a gun on the vehicle when the driver can just use theirs though. Although I realize there is difficulty aiming straight on some vehicles like the jeep (not flying vehicles). Since your view is locked forward, you can change the pitch of your gun and not be aware of it. In other words, you might mean to fire forwards and shoot your own jeep instead.
Anyway, you get more choices if you use your own weapons.
Of course, if you want to fire more then one weapon, or from a different location than the driver's seat, you would need to find some way to mount the weapons onto the vehicle.