Whichever bulb is behaving differently than the rest, probably has a burnt filament. Out of the 6 bulbs that light red, the outer 4 (2 on each side) have 2 filaments each. The inner 2 (one on each side) only have 1 filament each. Someone correct me if I'm wrong here as I'm going off personal experience only.
Well there are two filaments in the outer lights, so depending on which one breaks will determine it's behavior of how it lights. Ones brighter for the brake and blinking functions. One's dimmer for your running lights, like when you have your head lights on.
When you have your head lights on, you should see all 6 light up a little. Then when you turn your hazards on, you should see the outer four blink even more brightly.