Before doing anything, push/pull the hazard light button a few times, see if that get all the lights working.
Find the brake light switch at the brake pedal. Use a 12 volt test light, make sure you have power on the orange wire. If not, double check the stop lamp fuse.
If you have power, test for power at the white wire at the brake switch WITH the brake pedal pushed. No power? Either the switch is defective, or out of adjustment.
If you have power, check for power at the white wire of the turn signal switch connector with the brake pedal pushed. No power? There is a break in the white wire somewhere between the brake switch and the turn signal connector. Start looking for a spliced wire m, especially if a aftermarket brake controller has been installed somewhere in the past.
If everything checks out good to this point, press the brake again, check for power on the green and yellow wires of the turn signal switch connector. No power? Replace the turn signal switch.