Me thinks the problem has been found.
Yea so it turns out the rear brake light LED, the one I got it looks like the tabs were slightly smaller then the non-LED one. If you pressed it lightly into the socket, it was fine, but would come loose. If you pressed down to hard, it would pop back up and result in it only lighting up sometimes (apparently whenever pressing the Lock button only). I switched back to non-LED and the Bulb snapped right into the socket, nice and snug. It now illuminates when pressing the brake pedal.
The only thing I would like to learn though, is looking at the schematic wiring diagram, why does the brake lights come on when pressing the Lock button? Looking at the schematic, I would have concluded this to be not normal behavior.
Looks like @VOLVO-V70 solved it
Nice
Good thing you checked the bulb before rewiring the car.