How do you change rear brake light switch 2006 Harley Sportster?

It appears that HD changed the design of the switch from a relay type to a pressure activated switch.

Find the rear brake's master cylinder and unscrew the hex screw holding it to the frame. It is located on the left side of the bike, to the left of the starter cover plate.

Now, follow the brake line from the rear caliper to the "T" valve. Notice that wires come from the other side of the T. This is the switch.

Next, get a small clean container (I use a glad snack size plastic thingy) ready as there is going to be some brake fluid involved.

Pull the wired connector at one end of the T, loosen the bolts that are holding the other ends of the T and let the runout fluid drip into the container.

Put in the new one, leave the "supply" side of the T unconnected.

Repeatedly push the rear brake pedal until you get all the air out of the switch. Holding the brake pedal down once all the air is out, attach the supply hose.

Reattach the master cylinder and refill if necessary. Bleed the brake in the traditional manner. recheck fluid levels, and you should be good.

BUT, problems with the switch are often just air bubbles inside the switch. Try bleeding the switch as described above first to rule that out before you go through all the trouble of replacing the switch.