It is a well documented issue with the fox body. Ford mustang forum.

Mustang 5 0 Wont Start Ignition Problems
Fox body ignition switch symptoms. Fox 50 mustang tech. Electrical ignition toggle switch help. With age and cycling of the switch the metal crimps can weaken or even break causing the switch halves to separate. A failing ignition switch can cause system wide problems ranging from the ability of the car to start to maintaining speed and even keeping the radio turned on. It turned out to be the ignition switch. 1991 mustang lx 50 convertible.
You dont want your car to burn. If your 79 93 fox body wont start up on the first turn it may be caused by a faulty ignition switch. 302 gt40 heads gt40 intake 16 harland sharp roller rockers accufab 75mm tb bbk 75mm maf bbk cai 24 injectors f303 mac shorty headers bbk off road h pipe. These switches can go out over time due to wear or damage. Its been quite a few years since ive owned a fox body so i forgot about the common problem with the switch. Electrical mystery wire for ignition switch.
On the other hand if the switch is going bad the engine may keep running after you take the key out. Sn95 46l mustang tech. 86 50 fox body no crank issue fusible linkclutch safety switch fox 50 mustang tech. Problems when removing the key. Bad ignition switch symptoms. Mustang ignition switch many 1979 93 ford mustang fox bodies have experienced problems with their factory ignition switches.
If your ignition switch is worn out it may be difficult to turn or remove the key. They cause some very troublesome issues as they start to ware out wont start condition air condition ac issues or the worst case. Broken fox mustang ignition switches are a common problem. Dont be stuck with a mustang that wont start get your replacement today at late model restoration. Pick up a new ignition cylinder and switch screws to complete your install. Careful car owners can recognize the symptoms of ignition switch failure before the problem leaves them stranded on the side of the road.
Mine was literally in 2 pieces. Ignition switch fusible link. The problem centers on those few crimps that hold the two halves together. Fox 50 mustang tech. Forgot to update this thread.