Q: Brake lights don't work after replacing brake pedal switch

I can personally understand how frustrating it can be to try to get your brake lights working properly, and it looks like you have already put in a lot of effort to have it fixed! Having the information about your GMC Sierra’s other lights, including whether or not the brake lights on the cab are working properly, is very useful to help us tackle this problem. The working brake light on the cab informs me that the brake light switch is in fact working and that your Sierra may have a problem with the light bulbs themselves. Some bulbs are multi functioning and have two filaments inside of them. This allows the bulb to be both a tail light and a brake light at the same time and often times I have seen, only one will break. If you are comfortable tinkering with the bulbs on your GMC Sierra, I recommend going ahead and moving forward with replacing these bulbs and rechecking the brake lights to see if that addresses your concern. If for some reason the brake lights are still unable to work after replacing the bulbs, then I would highly recommend enlisting the assistance of a certified mechanic to properly diagnose why your brake lights aren’t working.