I have a 1987 f150 4.9 4x4.When my headlights are off the brake lights work but when i turn them on i have nothing from the tail lights.Changed the headlight switch, fuses now im stumped

 

Check to see if you are getting power to the taillights when they are not working. IF you are check the grounds. IF you have a good ground then its the bulb holders that are bad.

 

Did you inspect the plug / wiring at the headlight switch carefully when you replaced the switch? These had a HORRIBLE reputation for melting the wiring and plug connections at the switch. I am assuming that is the reason you replaced the switch, though?

Jim