as Rossmatt mentioned - there is an actuator part between the ignition lock cylinder and the rod going down to the actual switch near the brake pedal. It's been my experience that this part is junk - made out of pot metal - and will break. Cheap part - $10-$15 - but a PITA to replace.
The big clue as to this part's reliability is that when one begins to describe the symptoms to a Ford parts counter guy, one can barely finish the first sentence before he nods and goes to retrieve the necessary part.
1989 F150 XLT standard-cab shortbed 4x4, 5.0L, AOD, 3.55 gears, manual hubs, 31x10.50 tires, 218K miles. Repainted Oct09. SOLD - Jan2015
2014 F150 XLT standard-cab shortbed 4x4, 5.0L, 6-speed auto, 3.55 gears, auto hubs, locking diff, trailer pkg, 5K miles.