It's the hydraulic switch that's located behind your exhaust. It'll look like this
If you follow the hydraulic line that comes from the pedal, follow it back down the frame and watch where it goes up. It'll go into that switch. The pressure of your pushing the pedal down is what activates the switch and completes the circuit to turn the brake light on.
It's the same on all (or at least most Harleys) so they'll have it at the dealership. Might be $10-15 or so. Gotta remove the exhaust to get to it but it's not a big deal.
If you need help with it give me a shout. I'm in Lafayette.