Then people would complain that those from outside the ride and in the queue would see driverless cars and it would ruin the suspension of belief and Universal sucks and Disney would never do this.
Oh I don't mean to frost it so that nothing is visible. Just enough so you see the silhouette and shadow of the figure, and even its limited movement without being distracted by how unrealistic they look. You wouldn't even need to do the windshield or even the side window either as I've heard you rarely see the figure from those vantage points close enough for it to matter, probably just the rear window that guests can look through during the ride. Seems like a far enough fix to make without breaking the suspension of belief. Again though, I don't know if there is anything that happens in the truck cabs during the ride that would make that not very feasible.