How did they get the pick up truck close to him them? Put it at a stationary location that he usually drove by? Or remote/self driving? Or hired someone local that didn't know the full plot?
So most likely left there the night before (or even longer), they knew when he would be by (and it wasn’t going to be a one-off), and there you go.
The US Secret Service has had several folks who aren’t worried about consequences sneak through and severely injure - or depending on your pet theory regarding JFK - kill presidents.