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Ok, If you don't have a voltmeter, get one! Take the bulb out have a friend hold down the brake while it's running and put the positive end of the volt meter on the metal piece inside the socket, put the negative on a known ground, you should be getting a full 12 volts. Let us know what you get!
You can have a poor connection or the turn signal switch in column is also possible. The bulb socket can be rusted out or corroded, or a ground wire that should be at tail light to body somewhere close.
Step on the brake pedal and wiggle the turn signal switch, watch how the rear stop lamps behave. The stop lamps & turn signals both use the same filament in the bulb, which sometimes works and sometimes doesn't... wiggle the turn signal switch and watch it... parking lamps use the other filament, and there is a single ground connection.