I've just been trying to figure out how to get it work, so the time to wait is still set to a value that's not enough for loading/unloading.
Constant combinator stores the coordinates (X, Y) of a waypoint and the time to wait (T). It's connected to two arithmetic combinators: one outputs EACH -10, the other EACH + 10. From each arithmetic combinator I go into two deciders (one for X, one for Y) with a green wire. These four deciders are also fed via red wire with the radio signals. Each decider checks for one of these conditions: X<=W, Y<=V, X>=W, and Y>=V. Output is GREEN=1 if true. Next row is two deciders: One checks for GREEN=4 (meaning the current position is within an area of 10x10 around X/Y). The other is connected to the constant combinator and checks for B>=T. Both deciders output GREEN=1 if true. They are connected to another decider that is also connected to the constant combinator of the next waypoint; if GREEN=2, it outputs EACH (current value).
Now, the output of the last decider is X, Y, T of the next waypoint, and GREEN=2. I need another 2 combinators to eliminate T and GREEN. I've used arithmetic combinators (T *- 1, GREEN * -1) and another decider to combine input and change the wire. So X and Y are sent via green wire into the radio, while W, V, and B are read via red wire from the radio. I guess I could even save that very last combinator if I replaced the two arithmetic combinators with deciders (if X!=0 output value of X, if Y!=0 output value of Y). That would make it 10 combinators per waypoint plus 2 deciders overhead to sanitize the input for the radio.
However, this setup only works if the car already is at one of the waypoints! I'll have to manually set the coordinates with nothing but a temporary constant combinator connected to the radio before I can hook up my contraption. But once the whole thing has been initialized, the car should happily go in circles from one waypoint to the other. (Only tested with 2 locations for now, but it should work.)
