Do they use a script?
Or do they have cargo positions on the back of the tank. If they do, then something like this should work in each soldiers init:
This MoveInCargo Tank01
Yes its not a script, its actualy cargo positions, kind of like getting in the back of a 5t truck. It simply makes more sense to do it that way, and its also easier and works a lot better then using some sort of setpos/switchmove script. The only problem, however, it can't be done for all tanks, only ones that have special models and configs to allow for it.