I don't think exploiting software, in the usual sense of the term, is even necessary. Just box the truck in somehow when it's off the grid and can't phone home. There's no way the designers will build in the capability to ram through a barricade, will they?

My point, which I stated poorly, is that automated trucking is a whole new class of problems from a security standpoint. I'm leaving some scenarios out deliberately. I haven't even given much thought to the problem, but I hope someone does.