I had my rear door panel off trying to get to the third brake light. Dealer looked in their book and said it was easy just one nut hold the third light fixture in. Not what I found. There are four nuts and a bolt that hold the brow the third light is in. With them out I pried and pulled until I thought something might break. Still didn't get to the third light.
Tech at dealer told me they can't get much info on these rigs yet. As a last resort they refer to Fiat 500X info. Same car just a different skin.
Ok. I finally did it!
Like you, I tried to get to the actual light in the brow, but couldn't figure out how to get it off. With all the panels off the liftgate, I looked at the wires and thought about where they were going. The harness on the left goes up to the brow over the liftgate glass and the one on the right goes to the wiper, camera, plate lights, etc. I unwrapped some of the wires on the left to see what I was looking at and found that is has the water hose for the wiper and two wires. I figured these had to be the wires to the light, so I cut back a little shielding on one of them. Bingo! It was the power to the light; on with the brakes, off otherwise!
I stripped a little shielding off the wire (the lighter one), routed a wire between the panels and headliner, then up through the rubber boot, and soldered it to the wire. I wrapped it up with electrical tape and put it all back together.
I took the trailer out for a little test and everything worked great. Power and handling were fine, the brakes activated as they should. My hitch is too low. I'll likely upgrade to the bumper hitch, but it will work for this summer. I'll give some more detailed tow reviews when I take it out for a weekend.