I am GDI, using Sydney with Portable Ion Cannon, drove a Humvee into the AGT (just drive it thru the door), and manage to get myself stuck bumper-first between the MCT and the wall. A second Humvee came in before me and my Humvee got stuck even more. I exit the Humvee and when i got in again, Viola! , The humvee's turret were not moving when i moved my mouse around and i can look around the humvee 360 degrees. AND best of all i can fire my ion cannon!!! When i click fire button, both the humvee vulcan and my weapon fires, although i cant aim the humvee's one. AND my ion cannon had unlimited ammo (ive tested it too, killed a nod engineer with it).
When i exited the vehicle i still have full ammo for my ion cannon (but i cant recreate the effect again)