Follow the wire for the rear defroster into the harness through the rubber hatch grommet. From there you'll need to remove the passenger side panel and top panel to gain access just inside the headliner to reach the 3rd brake light wire which can be fed through the hole that connect to the original light. Fill that hole with whatever you want. I just cut the wires off and stuffed the original oem plug back in it.
Its self explanatory but since I've never seen anyone do this I figure I'd go into a little more detail. It's always difficult to execute a mod that's never been done before, even as simple as this one. From mounting to making sure it's still functional and also looks right in perspective on the vehicle with the rest of the lights.
Plus its winter here and I'm bored out of my mind.
Btw I made sure I had enough slack in the wires so if the light goes out or I need to change it I can easily unplug it by only taking off the inside plastic trim on the hatch. Otherwise if running the wire to the fullest extent the connector would be through the grommet and I would have to run the wire through it all over again.