Get in a jeep. This may work with other vehicles, but I've only ever used 
the jeep. Drive to an enemy's base (THEY MUST HAVE A VEHICLE REPAIR) Park 
on the vehicle repair, so that the vehicle is half way on the repair, and 
halfway off (MUST BE HANGING OFF THE NORTHEAST PART OF THE REPAIR)

Let the enemy destroy the repair. The metal beam/crane thingy will fall 
on you, trapping you. But, the game thinks that you are getting repaired,
so you are trapped, but invincible to ENEMY attacks. Friendly sarges can 
still kill you.