Brake light on one side doesn't work

Tony, our TB's only have 3 bulbs, 1 brake, 1 running, 1 backup.
If you happened to be playing with NEW LED bulbs (from local retail location) for the brakes, guaranteed you blew the fuse under the rear seat. It takes a special CK socket for the brakes that no one carry's. If you're only dealing with normal incondecent bulbs, I'd still check the fuse under the rear passenger seat.

Have you checked the fuses?

corroded socket? broken wire to the socket?

check park/tail fuse regarding dash ,park ,tail problem .brake light on passenger side is probably the brake globe needs replacing .Turn signals on passenger side could be globes or switch,check globes first ,if globes and fuse are ok ,may well be the switch especially if park switch and turn signals are on the same switch lever/stalk

The 4-wire flat plugs usually have a white (ground), green (right brake/turn) yellow (left brake/turn) and brown (taillights - left and right). If your brake and turn lights work but the taillights don't, check the plugs (trailer and truck sides) for corrosion, and chek both the trailer and truck wiring harnesses for chafed insulation and bare conductors on the brown wires. Likewiae if the taillights work and the brake/turn lights don't, perform the same checks on those wires. And last but not least, be sure that the white wires (truck and trailer) are grounded well to the truck body and trailer frame.

sounds like a bad ground

The first step is to check the bulbs in the tail lights. The brake light switch (on the brake pedal) is OK, as the center light works.

Check the fuses. And check for power at the taillights (These taillights have circuit boards on the back side that can go bad.) I don't think the hazard switch is it because the brakes would still work. Also check the brake pedal switch. There is a hazard flasher, does the hazards work?

If your brake light goes out when your headlights are on here is your fix:
1. remove taillight assembly (two screws hold it in place
2. Remove light bulb and socket from assembly (this will be the top bulb) put it to the side
3. Inside this hole on the rim you will see three silver metal contacts, two side by side and one by itself.
4. With a small screwdriver or a small pointed tool, gently pry up the contact which is by itself, all you are wanting to do is bend it upwards slightly.
Over the course of time this conact succumbs to the heat of the bulb and settles into the plastic. All you are doing now is making a better contact between the metal and the bulb.
This will fix your problem, it did mine and will save $200+ from a dealer.
GOOD FIXIN!