Fuses and bulbs MUST be checked with either a 12 volt test light, or a voltmeter. All too often; well meaning people look at a fuse or a bulb and call it good because the filament appears to be intact. This is not a reliable test; as bolbs and fuses can go open in locations which are not visible to the eye. This is why professionals use a meter or test light to check electrical circuits. Check that there is power to the high side of the brakelight switch; and that the power comes through the switch to the rear lights when the brake pedal is pressed.
If all the above tests are done and no problem is found; the ground wires from the rear light assemblies to the body ground point are either disconnected, or were never installed. This kind of problem can crop up as a vehicle ages.