Corroded or loose terminal. Bad ground.
Bad bulbs or brake light switch.
If the bulbs and fuses are fine, it must be your brake light switch attached to the arm on your brake pedal.
AnswerThe bulbs for the brake lights are usually separate from the normal tail light bulbs. Check and replace the brake bulbs first, and if they are still not working examine the fuses, switch and wiring.
Bulbs may be blown.
light bulbs
Likely to be the brake light switch on the pedal at fault if the bulbs and fuses are good and other lights work ok.
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.
There are a couple of reasons why a 2005 Saturn Ion's reverse lights will not work. The fuses may be blown or the light bulbs need to be replaced. Fuses and bulbs can be purchased at any auto parts store.
Start with checking the bulbs, then the fuses. In most vehicles the brake lights and siganal lights and running lights are separate circuits. If the fuses and bulbs check okay check the wiring feeding the rear of vehicle.
Check the backup light switch.
Does it have trailer light wiring also? check that too.
The dash light dimmer switch is turned off or broken.