I still think you might try controlling the car using characters entered on the onscreen
keyboard, as that seems to be easier than changing the sip'n'puff device between
wheelchair control and car control. Call up a terminal emulator that allows the PC to
talk to an Arduino in the car via XBee/etc, and enter S,F,B,L,R, etc. Also, a way to
command the Arduino to do anything in general.