The brake light and the tail lights on the right hand side share the same supply. It comes from fuse 7 (10A) in the Rear Power Distribution Box. If the tail lights on this side work, the supply to the lamp unit must be OK. If you don’t have volts where you should have, the problem is in the lamp unit, probably corrosion.
If you find you do have volts, as Neil says, the Rear Electronic Control Module supplies a ground to light the stop light when the brake pedal is pressed. It’s the black wire with the green tracer.