When attempting to order the vehicle with AII Programmable Vehicles it appears to only be able to rotate and won't move forward. Driving the vehicle manually still works. I suspect it may be due to how slow the Rex is but I have nothing to back this up at the moment.
Yeah that's the reason. The only way you can drive them manually is by speeding up and turning to start the motion. The AAI doesn't do that so yeah. And I can't be bothered to add support for it. Sorry.
