Take a motorcycle and park it next to a jeep so that the motorcycle's passenger seat is directly facing/touching the jeep's passenger/driver seat.
Switch seats into the passenger seat of the motorcycle by pressing the "A" button.
Get out of the motorcycle by holding down the "X" button.
If it worked properly, you should be inside the jeep, but not driving it or using the turret/passenger seat.
You can duck to hide from enemy fire, but you cannot lay down.
You cannot aim, and if somebody drives the jeep, chances are that you will be left behind, but sometimes you stay.
As you cannot aim I would suggest using a shotgun, bazooka, or support weapon.
Try using shotgun, and park the jeep in a place crowded with enemies.
Hide inside the jeep, then quickly stand, shoot, and duck.
They will die, but have no idea who shot them.
If someone gets in the jeep, whack them off. Shooting them will reveal your location instantly.
[more cheats for Call of Duty 3 here]