What would make my brake lights not come on, I've already changed the bulbs?

Those trucks have a common fault in the "multi-function lever" Its a bolt in unit: turn signal lever, wiper switch, cruise control. Try this, step on the brake and have someone see if the 3rd brake light is on. Light on? Most likely bad multi-function. 3rd not on? Most likely bad brake switch (mounted onto brake pedal lever)

I agree with all the above. The way to know if it is a faulty switch is if your turn signals still work. I'm sure there are websites that will show pictures of the switch located at the top of the brake pedal. Sometimes the pin is out of alignment, sometimes it needs needs only to be sprayed with WD40, and sometimes it needs to be replaced. All which you can DIY cheaply.

Might be a blown fuse in the fuse panel, I would check that first. After that consider checking the brake light switch located near the brake pedal.

First check for a blown fuse and then a relay switch. You might be able to find them in your operators manuel under the wiring diagrams. Otherwise check online or buy a repair manuel.

There is a switch mounted to your brake pedal; it could be faulty. Also, check the fuse for this circuit; it could be blown.