Is your car a GT with the fog lights? If so, that's your problem. Ford used too wimpy a wire for the fog lights which cause them to overheat which burns out the switch.
You can fix this by rewiring the fog light power to switch a relay and then have the power come straight from the battery to the fog lights through the relay. Use 12-14 gauge wire. I plan to do this soon, so if you want me to take pics, I will.
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351W 89 Mustang GT Convertible