This is a really easy job, but you will need to solder a bit. Make sure you are really careful and if you've never soldered before here is a cool tutorial. Start with "Car A". What you're looking for is the 2 wires that come from the circuitry to the motor. You'll want to wire in the rheostat inline according to this fancy diagram I drew below. Finish up with some electrical tape around your solder points then mount your rheostat somewhere on the car frame with more tape. Repeat this process on Car B, which should have the gearbox, motor and wheel assembly detached from the rest of the body. The only difference is you'll want to add a few feet of wire so that the motor and wheel assembly can be mounted away from the circuitry.