The Jeep thing is *supposed* to be a bit of "sleight of hand" where the audience is meant to *think* that it's the same Jeep that's exploding, flipping and crashing with the stuntwoman inside it.
But as you've mentioned, it's really a fake truck that's attached to the wall, and the switch is done out of sight of the audience. And it's not built so that the wheels actually touch the groun when it's rolling. Because the action seuqence happens so fast and there's so much else going on, many first-time viewers may not notice it, and think that it's the same truck. But repeat viewers notice the switch, as you have.
As for the plane, I don't know exactly how it's powered (though it's either electric or a lightweight engine), but it's definitely independently powered and steered by the person inside. I've been to a show where they really had to work extra long to get it set into its proper position. First they overshot the mark, then they backed up a little too much, then they finally got it in place. The person on the ground then locks the one wheel into whatever system they have on the ground. The plane pwoers itself around in a circle, but by locking it into the ground mechanism it allows the trapdoor to open at the proper time in the show when it's "wildly" spinning around in a circle.
There's a video on YouTube showing an incident during the show where one of the stuntmen came in contact with the propeller and the blades disintegrating on contact, as they're designed to do (they're really just lightweight painted wood). The show was stopped at that point, not sure how they re-started it from there.
EDIT: Here's the video of the incident:
http://www.youtube.com/watch?v=bYu6NBDzI3I
-Rob
