Do you know where on the circuit board are the connections to the H bridge?
What does the circuit board do? Is it the complete radio receiver and controller all in one?
What are you planning to use the Arduino for? You used the word "anonymous" in your first post, but perhaps you mean "autonomous"? Will the Arduino replace the radio receiver and control the car as a robot? If so you will also need to control steering and, presumably, detect the car's position.
Or could I wire the H bridge from the circuit board itself?