In the shoot from the truck level 5, the car stops, and I don't know what to do.

Just after the grenade launcher pops into my hand, blow up the car, and drive over a river. The driver says something about a helo. Then the car stops.

There is no one I can see to kill, and I don't know how to get out of the car, or get it moving again.

Is it a crashed game? Am I missing something? What do I do? Or is there a cheat to get to level 6 as I don't like the moving vehicle levels anyway?