As far as I know they are not allowed to use GPS or transponders. Even though they have them aboard, they are only used for the race officials for location, and in the event of a crash. Only roll charts with distance and direction - left / right are used. They are given the entire route for each stage before the race begins. But, updated additional (secret) check points and course changes are given out 1 hour before each stage starts. That way they have to recalculate direction, locations, fuel, and strategy. It does keep it interesting, difficult, and entertaining to watch.
With that said. Imagine someone handing you a map that said drive 1.7 km, then 45 degrees right, then 32.8 km, 90 degrees left, and so on… for over 500 miles in a day, over rough terrain. On top of that, cars are allowed a navigator/ mechanic, unlike bikes. ???