I was getting water out of my tailgate when I raised it. Searched for the leak all winter. Finally I found that water was getting under the rubber seal somewhere (probably need a new seal), running down to the bottom of the seal and then in through those small drain holes on the bottom edge and into the tailgate. Sort of a reverse drainage. Some of this water remaines in the tailgate and then runs out when the tailgate is raised. I covered those drain holes with plastic electrical tape to see if that was where the water got in. Sure enough, the tape stopped water from getting into the tailgate. Some water accumulates in the rubber seal at the bottom but that drains out as I start to raise the tailgate. The inside of the tailgate remains dry.

If I can't replace the rubber seal, I may try taping something on the lower edge of the tailgate opening to hold the rubber seal open slightly so the water can drain out.