Sometimes the battery has enough juice to run the accessories but not enough cranking amps to start the car. Check your battery cables and if you're still convinced that it's not the battery then the only other thing I can think of is the neutral safety switch if you're an auto tranny or the clutch safety switch for manual. Maybe it's faulty.
Edit: BTW, I assume you also checked the fuse in the SJB. Ford uses a slow blow fuse so unless you look really close you might not think its blown.