No heat ac or horn check all fuses and change the switch that control the hi low

What is your question?

i have a Ford E 250 99 my problem is the heat or AC and the horn does not work when I bought the truck the owner told me that if I'm running the heat or AC not to use the horn because it will blow a Fuses. so i change the switch on the heater to control the high and low and still no heat I checked all the fuses and they were good so what I'm trying to find out is why I can't used both of them the same time.