I know its possible that you can only man the turret of an aat.
I saw that with other vehicles in maps like rise of the empire (gunships)

Now I only removed the pilot position, but it doesn't change!
I could man the pilot positon and drive with it.

So what does I have to do that I can't move the aat and only man the turret of it?

So you want to:

1-Have no way to drive the AAT, only to use the turret
2-Have the the AAT drive itself, only allowing you to use the turret
3-Only be able to drive the AAT, having the turret operate itself

I'm assuming #1, but I want to make sure that's what you want.