Okay so my drivers side hatch rear brake light brightens when I touch the brake pedal with the lights out but when the lights are one and I press the brake pedal it doesn't light up. Changed bulb same thing. CONFUSED
I had a problem like this a little while back with both the passenger side brake light and the one on the hatch of my 03 outback. The problem was mainly due to moisture and corrosion in the sockets that the bulb plugs into.
You could also have a small break in the insulation of one of the wires the feeds up through the hatch door. I'm not sure how comfortable you are with a multimeter but it easiest just to visually inspect the socket first, and if it looks a mess replace the socket, especially if you have been blowing fuses.
I got the sockets I needed by clipping a few out at the junkyard and then installed them in my car with a few butt connectors and some electrical tape.
The root cause of my problem was a thin crack across the top of my taillight housing that was allowing water to collect inside and then short out and corrode the bulb sockets.
After sealing up the crack in the taillight and replacing some sockets, I haven't had any issues with my brake lights. It cost me about $5 to fix.