Is water getting in the headlight? A little splash on a hot bulb and it will blow out. If you are finding the bulb physically shattered, this may be your cause.
Good clue... The headlights will also fog up with moisture that can drip on the bulbs. The bulb may crack but not shatter. Either way, the results are bulbs that don't work.
Before replacing the multifunction switch, check the switch wiring. 87-93 Mustang GT have known problems with the fog lights causing the headlight and multifunction switch wiring to overheat. The ground for the headlights is another problem area. The grounds frequently are loose, corroded or missing.
SEE
Mustang GT Fog Light Fix to fix the fog light problem. The stock wiring isn't up to the job and is overheating. The headlight switch & turn/multifunction switch are affected by Ford's wiring problem. Sometimes it overheats so badly that the plastic shells of the wiring connectors start to melt. This will show you how to add a relay to the fog lights to relieve the overload on the headlight wiring.
Multifunction turn signal switch:
Before you think about replacing the multifunction switch with one from the junkyard, here's some things to be aware of...
The problem is more common in GT models because they had fog lights on the same power wiring as the headlights. Ford undersized the wire and that caused problems.
A word of caution about multifunction switches is in order here. The multifunction switch (high/low beam, wiper, turn signals) are different for different years. 87-98 will work in any 87-89 car. The 90-93 switches only work in 90-93 cars. You can't put an early model switch in a late model car, nor can you put a late model switch in an early car.
Supposedly you can move the pins around to make the switches work in model years that are different from the car the switch came out of. I cannot verify that and haven’t tried it.
Other possible problem sources for the turn signal & headlight malfunction are the ignition switch, multifunction switch and the plastic shell that holds the turn signal wiring connector pins.
