I just did this yesterday. Remove the trunk liner. Then insert a long flathead screwdriver under the passenger side of the plastic piece from the back of the vehicle where the weatherstripping overlaps it. Pry up on it and it will pop and loosen, then work your way to the drivers side, prying it up and remove the whole piece. You will see that it clips into the side pieces and the frame. Once done, I stuck a coat hanger in from the corner of the vehicle and used duct tape to attach the wire from the LED to the hanger, then carefully pulled it through so I could tuck it into the trim piece and route it up to the inside of the OEM light mounting.
It just takes time and patience, but it's not hateful. MY problem was getting the 3M tape to stick to the inside of the trunk. It simply would not stick, even though I cleaned and degreased it with All Purpose Cleaner, so right now it's attached with duct tape (which DID stick) until I can find a double sided strip of adhesive that works.
