if the bulbs are good, check the fuses to make sure the b/u lamp fuse isn't blown. If the fuse is good, then you should inquire about a back up light switch for your particular make and model vehicle, as this is the most likely cause.
you got to replace to light bulbs
The reverse lights automatically come on as you put the car into reverse , if they are not coming on your most likely problem is the fuse or bulbs
you could have possibly bought the wrong bulbs and/or fuses. or when you were replacing them you may have damaged the wires. Also the reverse light switch may be defective.
check your bulbs
did you check the reverse switch on the trans ?
the power source
i changed the running light bulbs (3 of 4 are out) on Chevy 2005 trailblazer, and they still don't come light up?
Not necessarily. They can be brighter, or less bright. Both the energy saving light bulbs and the old-fasioned incandescent light bulbs come in different powers.
u have a short in ur wireing.
my guess is that it is the brake relay
You have a faulty ground to one of the tail light bulbs and there is a reverse current flow back through the brake light circuit. It could be either front or rear turn signal light.
I can assure you that light bulbs come in many different shapes.