This would indicate that you have a problem with the fuse, it should have tripped or melted the link, lucky not to have a fire. The first thing to investigate is the why did she not trip. Unfortunately a lot of guys "restore" a car and leave the electrics completely untouched, depending upon how nice the car is or what your plans or budget will allow I would up date the system completely on this car. While an old model A may not need a wire upgrade and it is preferred not to have one I have always felt that the wiring on a Mustang was never its strong suit in the first place. A lot of the stangs I have owned in the past I have used a remote fuse box and run accessories off of that. I may have a mustang wiring book or you can get one at most of the early stang parts houses, a good investment.