I'm not sure if anyone has done this before, but there are five wires leading from the wheelchair joystick into the voltage controller for the motors. Five pesky wires.
I don't need a motorshield because the controller handles all that voltage for me. But what about these wires? I know I need to hack them, but I don't know which does what.
Just a guess, but your 5 joystick wires will probably be:
1) Power in
2) Right motor forward
3) Right motor reverse
4) Left motor forward
5) Left motor reverse
Sorting which is which will require some trial and error.