I don't understand where the problem is. The SAG driver will use a navigation system. For the sake of simplicity, assume Google maps on an Android device. The driver will therefore see "his/her" route and location, to which you will want to add a marker showing your position (a feature called location sharing) and perhaps make sure that the SAG's route is the same as yours - the simplest solution to ensure that routes are identical would be to share your map, including the directions layer. OTOH, you may not want to bother with this unless both routes are considerably different (let's say that you break down on a trail or dirt road, whereas the driver uses a highway, it'll be very simple to reroute the driver to the nearest access point).

So - share location and let the driver fiddle with routing.