OK I finally managed to replace the third brake light on the 2019 Forester. I probably should have made a video but it wouldn't have been very pretty - too many missteps and I almost broke the spoiler (nothing a little silicon caulking can't fix but it was close). But just to set the record straight -
(1) To get to the tail light you have to totally remove and disassemble the spoiler. This is not for the faint of heart (see my note about breakage above) so I don't recommend it. You might be better off replacing the entire spoiler assembly, which is more expensive but easier to deal with.
(2) To remove the spoiler you need to remove 6 screws which are at the top of the inside of the rear liftgate. You need to remove some of the inside trim to get at them, but two of them are covered by grommets. You'll need a 10mm socket with an extension to get them off. After you get them off there are also some plastic clips. (this is where the video would have helped).
(2a) After taking the spoiler off the liftgate you'll have to unplug the wire leading to the brake light.
(3) To take the spoiler apart - well, I'm not entirely sure because that's where some of the breakage occurred. There are six Phillips-head screws that are very far inside the spoiler - you'll need a very long screwdriver or an extension to get at them. Then there are two 9mm nuts that hold the brake light assembly onto the spoiler - they're also very deep inside the spoiler so again you'll need an extension to get at them. Then there are two Phillips head machine screws that hold the sides of the spoiler cover onto the spoiler body. Those also have to be removed.
(4) Once all of those screws and nuts are off you can sort of wiggle the spoiler assembly apart. I think. As I said there had been some breakage so things might be different for you.
(5) Pop out the old brake light assembly and put in the new one, threading the cable the same way it was before (you did note the cable routing, right?).
(6) Loosely reassemble the spoiler assembly, then put back the screws and nuts - I started with the two nuts holding the brake light assembly, then the six Phillips head screws that hold the body to the cover, then the two machine screws holding the side of the cover to the body, then I put the spoiler back on the car, reattached the brake light wiring, then reattached the hex head screws inside the liftgate. Note that magnetic screwdrivers and nut drivers and hex drivers are just about required for this because the screws and nuts are located pretty far inside the assembly - I actually lost one of the screws inside the liftgate and had to find a replacement.
