I had the same problem with my turn signals on 1997 f150 that
alot of other people had.
I had to clean the green gook off the turn signal switch.
Before I started the turn signal were acting funny, but I had no
back brake lights. The center brake on the cab works.
This is my Son's truck home from college and he had to take my truck back to school , so I need to get this fixed so he can return my truck.
thanks
Mark

 

Make sure that the flasher switch in the center of the steering column is all the way up. If the flasher switch gets gunk under it, it will not come all the way up to the off position. In this case, the third brake light will work, but the rear brake lights will not.

Hope this helps,
Chuck