Welcome to the forum

The Turn signal and Brake light use/share the same filament in the tail light bulbs.
Do the turn signals work in the rear?

If so then bulb is good, so is the Ground wire and wire from multi-function switch.

This would indicate problem in the Multi-function switch, turn signal stalk
But just have a look under rear bumper area for DIY trailer hook up, if you see a "black box" that is a converter and could be bad, it shouldn't effect trucks lights but.................

Brake light fuse is #19 20amp
When you press down on brake pedal 12volts goes out to Cab brake light and to the multi-function switch, on a Light Green wire, if fuse #19 is good then cab brake light comes on
If turn signal is not on then multi-function switch sends out that 12v to the left and right bulbs, both brake lights come on

If turn signal is on then the steady 12v(brake light power) on that side is cut off by multi-function switch and Flasher 12v is sent out on that side to the bulb.

Diagram below, it looks more complicated than it is, lol
Fuses #38 and 42 are for trailer lights not trucks brake lights

Pull out the multi-function switch and use volt meter, or test light, to see if the Light Green wire has 12v when brake pedal is pressed down, if so then replace multi-function switch, if not then you will need to trace that Light Green wire back to Brake light switch

 

 

toggle your hazards on and off as well - they hardly get used and can cause some wonky issues.