Three switches,
1. Torque converter clutch cutout; disengages the TCC when the brake is pressed, cuts 12V from going to the transmission
2. Brake lights; cuts power to the brake lights when the brake is released (or supplies power when the brake is pressed, depending on how you look at it)
3. Cruise control cutout; Cuts power from the cruise controller module as well as cuts power from the Vacuum solenoid for the Cruise (for redundancy) when the brake is pressed.
Specifically, if you say the wire is purple, then I am pretty sure it is the top one on the list, The TCC cutout.
[This message has been edited by Fierobsessed (edited 08-31-2007).]