Getting the RC car ready for this task is not difficult and will take you less than half an hour to do. You will no be taking the car itself apart, you will just be focusing on the controller.
Step 1: Remove the fixing screws that hold the controller together.
Step 2: Locate the fixing screws for the circuit board and remove these, be careful not to disconnect any wires or damage the control board.
Step 3: On one side of the board you will see where the joysticks for controlling the board are located. These are normally very simple switch which just make a connection when pushed one way or another. You will be able to see how they work by moving them back and forward. You can see the contact points highlighted in the picture below.
Step 4: Each joystick should have three connections, one in the center directly below it, one when it is pushed forward and one for when it is push backward. Make yourself aware of their position front and back of the circuit board. You will probably want to leave the controller so that it still functions correctly so figure out which side of the board is best for you to solder your wire to that will allow the controller to continue to work properly and will allow you to fit your wires easily around the inside of the controller. The removed board is shown in the picture below:
A close up of the connections for each switch is shown in the pictures below.
Step 5: If your happy you know how your wires will be positioned so that you can put the controller back together then you are ready to solder your wires on. You should choose three different colours and cut two of each colour making sure that you have enough wire to get from the joysticks out of the controller to your interface board. The connection in the center below the joystick is the earth so you will probably want to make that one a black wire or something obvious like that. The other two are not as important as you will be able to test them by turning your RC car on and connecting one of the wires to the ground.
Step 6: Solder your wires to your switches.
Step 7: Make a hole in your controller casing to allow the wires through.
Step 8: Feed the wires through the controller casing in a similar fashion to that shown in the picture below.
Step 9: Screw it all back together and your controller will look something similar to the one shown in the picture below.
Step 10: Connect to your board and enjoy.