98% sure it will be the brake light. If it was me, I would drive to the dealer I bought it from and ask them to fix it. If they say they are booked up and can't do it for a week, fine, have them store it inside until they can fix it and give you a loaner in the mean time. My guess is that you will be rolling out of the service bay that day with a new 3rd brake light or at least a new gasket for the light. You might casually mention that you wouldn't mind switching straight up with another 2017 sitting out on the lot if they want to take their time with fixing that truck.
I am all for being calm and reasonable in dealing with these kinds of issues, but the fix for your problem doesn't involve removing the heads from the motor or anything like that. It is a 15 minute diagnosis and 10 minute fix. Maybe a day if they have to order a gasket or complete light.
Now if replacing headliners and seats is necessary, then I can see a bit more time being needed. Either way, I wouldn't have my brand new $50,000 truck out in the rain, letting even more rain into the interior.