The wiring is not too hard. There is just one important step. Basically, you are just putting the motor controller “in series" with the wires that connect right onto the motor terminals. You don’t have to worry about any other wiring. However, physically mounting the motor controller is a little more challenging, and you probably will need to splice some wires.
Here are the steps:
1) Confirm the polarity of the motor wires and terminals
This means confirm which wire attached to the motor terminals is positive and which is negative, because it is probably the opposite of what you would expect. This is the important step. You will need a multi-meter and a helper.
With the multi-meter set for around 20 volts, put the test leads (red and black) on the motor terminals. (Either lead on either terminal is OK to start.) Have the helper tilt the car and start it. Read the voltage. If it is negative (a minus sign (-) on the display), the test leads are backwards. Try it again with the test leads reversed. When you see a positive voltage, mark the wire and terminal where the Red test lead is connected with a + sign. That is the positive terminal on the motor, and the wire going to it has the positive voltage. (I usually mark this with masking tape so I don’t forget.) The other wire is negative voltage, and the motor terminal it connects to is the negative terminal.
One note. If you have any electrical experts around, they may be surprised to see that a red wire shows up as negative. But yes, this is the way the factory sometimes wires them and it is why you have to check.
2) Mount the motor controller
This can be the hard part. I will give an example for the RioRand controller. If you use something different, you will have to tweak this. On the older McQueens, there is usually room for the controller on the floor of the motor compartment on the opposite side of the center post from the battery. The controller board has four holes drilled in the corners. I usually attach the motor controller to the floor of the car with #4x3/4” pan head sheet metal screws. However, don’t screw the controller down directly to the floor. Use standoffs between the controller and the car floor. I usually just use plastic craft beads. Then flip the car over and put thermal glue on the screw ends where they poke out below the car.
Then, there is the controller potentiometer (speed control). You want to mount it somewhere on the car body where the child can’t bump it. But the leads are only about 7” long. Usually I will mount it on the top side of the car just behind the rear seat edge. Drill a hole through the car body where you want it to go, then a second hole inside the near wall of the motor compartment so you can run the leads back into the motor compartment through that hole. The leads have a connector that you can pull off the motor controller so you can thread them through after you mount the potentiometer.
Note that the potentiometer has a small tab sticking up on the front face. You have to drill a second tiny hole near the first mounting hole you drilled. The tab has to go into the new tiny hole. The tab prevents the potentiometer body from twisting. If you don’t drill that second hole, the potentiometer will mount crooked. (I realize this is hard to visualize just from my text, so if you want a picture let me know.)
3) Wire the controller
Cut the positive wire going from the other wiring to the positive motor terminal right in the middle. Strip the end still attached to the motor and screw it into Motor+ on the controller. Strip the end of the wiring coming from the rest of the wiring, and screw it into Power+ on the controller.
Then cut the negative wire going from the other wiring to the negative motor terminal right in the middle. Strip the end still attached to the motor and screw it into Motor- on the controller. Strip the end of the wiring coming from the rest of the wiring, and screw it into Power- on the controller.
Depending on where you mounted the controller, the wires might not easily reach the controller. Don’t stretch them. You might have to splice some extension wires into the existing wires. Let me know if you need some suggestions on that.
Finally, glue the wires attached to the motor controller to the board and green terminal block with thermal glue so they don’t loosen.
Hope this is helpful. If you have any questions or need pictures, just let me know.