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You may have a bad ground at the bulb or in the taillight wiring. This can be caused by a bad or improperly installed bulb, a broken wire, a broken connector, or corrosion at a body ground point.
Here's the the scenario for a bad ground. The taillights are on and the brakes are applied: The taillight goes out. Reason: With a bad ground, the taillight finds a ground through the brake light filament in the same bulb. When the brakes are applied, that ground is taken away because there is then 12 volts on both sides of the tail light filament, so it goes out.
As a general rule of thumb, when you have a really wierd, puzzling problem in any electrical circuit, look for a bad common (or "ground" or "earth") connection.
Here's the the scenario for a bad ground. The taillights are on and the brakes are applied: The taillight goes out. Reason: With a bad ground, the taillight finds a ground through the brake light filament in the same bulb. When the brakes are applied, that ground is taken away because there is then 12 volts on both sides of the tail light filament, so it goes out.
As a general rule of thumb, when you have a really wierd, puzzling problem in any electrical circuit, look for a bad common (or "ground" or "earth") connection.