Broken wires in the headlight switch. I do a few of these per month. Had a BF in with the same fault 2 weeks ago. Everytime you turn the blinkers to the left, it stresses one or more fine wires in the combination switch. You can repair or replace. To confirm, drop steering column down, remove covers so you can see the blinker switch at it's base. Next pry the small cover from the rear of the switch. I usually use a pick tool and dig in at the headlight end of the stork. You will see 5 fine wires and some black tape near the base of the switch. The wires break near the tape. If you are great at soldering, you can cut and splice some fine wire in place of the broken ones. Fit heatshrink and test, then refit covers. The other option is to disconnect battery, remove airbag, clockspring assy and fit a new combination switch from Ford. Used switches will have the same fatigue issues.