Both sides or just the left side? If left side only, make sure the rubber cap for the trunk wiring is properly in place (like here at 2:00
https://youtu.be/O1ruROdlZkk )
If it's rather on both sides, see if there's a problem with the 'vent' grids attached to the rear fenders (from inside, behind those side trunk covers). I'm yet to hear about the back lights letting water in, unless they have already been removed for some reason at some point, and not put back correctly.
While there, pull the battery and try removing the black cover of that piece with battery cables, it might have been corroded inside and may cause problems. Also look with a flash light at the Rear electronic module (left side) for any water leaks on it