I actually finished this last spring but never made a video of it.

It is controlled by an ATtiny85 programmed in Arduino code. It reads the pulses output from a hobby radio (normally controls servos) and converts them into voltages from 1 to 4 volts using pwm and a low pass filter, mimicking the original joystick.

The only problem with it is that sometimes the wheelchair controller will go into error mode because it thinks that something is wrong (like if the joystick has become detached). This is probably the fault of the very cheap switches that I use to change between joystick and radio control.