Why doesn the tailight change brightness when I depress the brakes?
Re-install your factory taillight and if it does the same and it seems very bright then the problem is with your rear brake lever. What is happening is your brake light is stuck on. Your rear brake lever has a button that either pushes out or pushes in to activate the brake light. Many times, the rear brake lever does not fully engage or dis-engage this switch; therefore never shutting off the brake signals to the taillight. Check your rear brake lever and make sure the switch is working properly.