Since everything works well enough for the center (window) brake light to work, you can eliminate everything in the circuit until the wire harness splice splits off into two different legs. Both of the brake light circuits use the same switch, fuse, etc.
If both corner lights are out (and assuming the new bulbs are okay), the first thing I would check for is a bad ground circuit. If I recall correctly, there is a ground circuit eyelet screw located right off the harness next to the rear corner light. This ground is separate from the ground used by the lift gate.
The next thing I would look for would be a broken splice on the positive + side of the circuit.
Hope that helps
-Rick
