[Foryota]

personally I would not drop headliner on almost brand new car. If the antenna broke, shouldn't it be covered under warranty? If not or just want to upgrade, see if you can get access to wires by removing rear light dome. If I couldn't do it that way, I would probably run new wire under the carpet to the back and install inside spoiler or even run it along luggage rack on the roof (if you have one).

You may have missed the last sentence in the original post. He backed into a garage door, so the fin antenna is damaged. Could be allowing water in through the hole.

OP, if you search for “mouse”, there’s one pic from a user who had to drop the headliner of his 4.3 due to an infestation. While the antenna is different, I’d think they didn’t change the mount system for the unit. If the access mentioned in post #3 above doesn’t work, you’ll be into pulling the rear side covers I’d bet. Good luck!