Tail/brake lights not working?

Hello, does the 3rd Brake light work? The one centered at the top, high mounted Brake lamp? And the Turn signals, do they work? Just looking through the wiring diagrams to determine the setup your vehicle has.
And when you say you tested the sockets, did they have power during testing? Or light a test light? If you used a meter or test light and had voltage at the sockets, and you used the body or frame as a ground for the test light with positive results then it sounds like there is a bad ground for the brake/tail light assembly. But more info is needed on your testing procedure.

https://www.2carpros.com/articles/how-to-use-a-voltmeter

https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester

Here is the wiring diagram for the Lamp assemblies in the rear. If there isn't a bad ground issue, there might be a problem with the fuse panel that feeds the rear lamps, but without knowing how and what you tested it's difficult to say what exactly the issue is. The Brake lamps are on a completely different circuit than the turn signals except for the fuse panel, ground connection, or any bulk connector in series with the rear lamp assemblies.

When you used the Fluke meter to test the sockets did you have 12volts there? Also, 12volts with the Brake pedal depressed? To be honest with you, I'm going through the aftermarket wiring diagrams as well as the OEM Subaru diagrams, and I'm not sure either are correct. I used to actually work for Subaru and owned a 2005 Legacy, it was a great car. But these diagrams are showing the rear lamp assembly with turn, stop, and clearance lamps in each rear lighting assembly. So, 3 bulbs in each side, then there are turn signal LEDs in each side mirror, then the license plate lamps, but then just the Front turn signal lamps, I'm not seeing any upper lamps that you're mentioning. Can you take a picture for me and post it here? I even changed to an 06' and still the same. Does your vehicle have 2 sets of brake lamps on each side, one on top of the other?

Looking at the 2006 OEM Subaru diagrams, I'm seeing a completely different setup.
This diagram below is from a 2006 Legacy, I'm sure it's not much different from the 05'. But the build date of your car might be later in the year. There is a tag on the driver's door opening that will have a month/year that should be the build date. But even with this diagram I can't tell which would be the upper brake lamps and which is the lower, they have all the feed wires to the bulbs labelled as brown. And the Ground for each as black.
Below are the diagrams I could find that have two sets of bulbs, the 2005 diagrams do not, or they are incorrect.