Trent,
Just did this on my softail. If your sportster is like my softie, the rear brakelight switch is located aft of the rear master cylinder along the lower frame. Follow the hydraulic lines to locate the switch, it is an 11/16" fitting with 2 wires attached. Pull off the wires and get a thin wall socket with extention (a wobble extention worked well) and remove. On the new switch you will want to wrap it in teflon tape prior to installation. Reconnect the wires. You will need to bleed the system after this. Good Luck!