I searched only to find the same problem but no solution.

headlights just quit working when the switch is turned on.
I can pull back on the turn signal switch and the high beams will come on till I let it go.
Fuse #8 under the hood is good.
All other lights inside and outside work as they should.

 

Both low beam are not working? Can you visually check that the filaments in both are good? There should be two fuses, one for low left and one for low right for fail-safe. Do a voltage test on the bulb clips as well with a DMM.

 

I removed the switch assembly and jumped the contacts for the headlights. wiring, fusees, headlight bulbs, are all fine.
took the switch apart found the problem and fixed it, they work fine now.

 

You found and fixed the problem all by yourself! I am so proud of you!

 

I would like to to all the grate f150 experts of this site for sharing just a little bit of their wisdom to help me with a problem.

 

I know now this is an old thread; however, which switch did you take apart the turn signal arm? I have the same issue and would like to fix it myself.

 

That would be the Multi-Function Switch - sometimes called the MFS.