The way to sort this out is to turn on the headlights to high beam and then turn on the hazard flashers. See if all bulbs on both sides in the front and rear flash normally. If all the front and rear bulbs flash normally; the problem is not in the lights or their wiring. If any bulb is dim or is out; it may be defective, or installed improperly, or else there may be an open circuit or resistive connection in either the power wire or the ground wire or ground connection to that light assembly.
You'll need a voltmeter and/or a 12 volt test light to check these wires. Measuring the voltage between the power side and the ground side of the bulb while it is lit by the flasher will be the most sensitive test. If any bulb has signiifcantly less voltage being dropped across it than the others; this indicates a resistive connection either on the power side or the ground side of that bulb. If a bulb has less voltage across it than the others; check the voltage between the ground side of that bulb's filament and a known good ground point. If there is any voltage reading in that test; there is a bad connection in the ground wire. Many light assemblies have a dedicated ground wire attached, which goes to a bolt in either the frame or the body. If a ground wire is disconnected or broken; or its connector is corroded; that can cause this type of problem. Other vehicles do not have a separate ground wire from the light assemblies, and instead ground the lights through the mounting bolts for the housings. If a car has been repainted; or if there is original paint overspray in the areas underneath the light housings, this can block the ground current from flowing. The body should be filed or scraped down to bare metal in those areas. Sometimes a dedicated 12 gauge stranded ground wire will have to be added to housings which do not have one.
Occasionally, on vehicles which have corrosion in the joints between body panels; the dedicated ground wire for the lights may have to be run through or underneath the vehicle all the way up to the battery ground connection. Once all the bulbs flash normally; if the brake lights still do not work; you should confirm that power which is strong enough to light a test light is available at the brake light switch. If no power is in the wiring at the brakelight switch; the fuse probably is defective EVEN THOUGH IT LOOKS FINE TO THE EYE. Replace that fuse with a new one of the same rating. If you cannot get power from the fuse block for the brake lights; it may be necessary to tap into another hot power source, install a 15 amp fuse in an inline fuseholder, and run it to the brake light switch.
If there is power at the brakelight switch; but no power at the brake lights; the turn signal switch may be defective. (Power from the brake light switch then runs through the turn signal switch).