I had this same exact problem.
For me, it was a wiring issue. Open up your trunk and on the left side, you have your wiring harness that runs through. After repeatedly opening and closing the trunk, it can wear down and fray or even completely snap wires. So split open the protective rubber hosing and inspect your wires at the point where they are bent repeatedly. You just might find a frayed or split wire.
Hope this helps. I have a really good feeling that this is your problem.