I installed the Curt wiring kit on the CX-5
The "trick" to getting the rear panels loose, is the "hidden" bolt, located under the spring loaded tie down points,..... just pop open the bolt cover w/ a jewelers screwdriver.
I used a sturdy jewelers screwdriver, and covered the tip w/ some electrical tape.
The panels just pop off, with the help of the screwdriver.... after you have enough space to get your fingers under the panel, (and after removing the hidden bolt(s), you can just pull the panels loose.
Under the car, on the drivers' side, you will see a round "plug" that you can cut a hole in, (w/ a razor blade or exacto knife) and run your connector thru the removable plug. I put some vinyl tape on the round plug after the wires were run thru it, to insure that no water or dirt could get into the car. The connectors on the Curt kit can only connect one way (the connectors are each slightly different), so there is now way you can connect it wrong.
The panel that runs across the cargo area will just pop off, .... just pull up on it. You can't see it in the pics, but there is a brass screw on the drivers side (behind the panel) that you can connect the ground wire to.
When you put the panels back in place, use the small screwdriver to make sure the rubber material goes back into place properly. The pics are kinda big.... I guess I should have shrunk them down, before posting.