I grabbed the handbrake from a car at a local wrecker, the switch was still attached. The switch is for the dash light that tells you the hand brake is on, most cars should have that switch, if it's not on the handbrake try a different car.
A spring was added for resistance and it was wired to a separate 3rd party ps3 controller so it could be used in GT5... but not on the PC. There are instructions floating around on how to hook it up to the G27/other wheels though.