Woah, sorry, I missed this one for about a month.
I would hack up a super cheap RC Car, like one of the ones that goes forwards whenever it's switched on, then backwards and turns when you pull a trigger in the remote control. The turning while going backwards is purely mechanical, but you do have control over the direction that voltage is applied to the motor. Throw in a big diode and a resistor (and maybe a transistor as a switch if necessary) and you've got zero volts most of the time, and battery voltage when you pull the trigger.
If you want to interface the transmitter side to some electronics it's most likely just a mechanical switch that you can replace with a transistor or relay.
If you want to get more creative you can pick up a car (or boat!) with a few more directions of control, just stay away form proportional steering!