The tail light/brake light bulb has two filaments. If one of the filaments is broken, you will get either only tail lights, or brake lights. If a new bulb doesn't work, then you have a broken wire or bad connection in your plug-in.
You have a poor or no ground between the vehicle and trailer.
Look for a fuse labeled TAIL LIGHTS. There are 2 separate circuits, one for brake turn and flasher and the other is for tail lights.
The brake lights and the tail lights are separate circuits. Check the brake light fuse, then check the brake light bulbs, then check the brake light switch.
Brake lights only come on when you press the pedal. The brake light is either a different bulb or filament in a dual-filament bulb than the tail light.
It could be a blown fuse. Tail lights and brake/turn are on separate fuses. Apparently there is not a ground problem since brake/turn works OK.
Check the switch on the brake pedal
On a toyota tacoma 2003 SR5 when the tail lights are turned on the brake lights come on and when the tail lights are of the brake lights work just fine what could be the problem. If the truck has a trailer plug, inspect the plug and plug wiring for damage and corrosion.
Check your fuses.
its on different bulbs
The tail light bulbs are on a different circuit than the signal light/stop light bulbs. Check for a burnt fuse or a loose wire.
on some cars the tail lights and brake lights are totally different bulbs gain access to the bulbs and have someone push the brakes so that you can determine which bulbs are the brake lights and replace the tail lights