You have a short circuit. That fuse feeds a lot of circuits but you already have a major clue staring at you. We know it's related to whatever the AC / heater system is controlling. The logical first guess would be a shorted AC compressor clutch coil. Unplug that, then see if the fuse blows when you switch the AC on.
If it does, a simple trick to finding a short is to replace the blown fuse with a pair of spade terminals, then use small jumper wires to connect them to a 12 volt light bulb. A brake light bulb works well. When the circuit is live and the short is present, the bulb will be full brightness and hot so be sure it's not laying on the carpet or against a plastic door panel. Now you can unplug electrical connectors and move things around to see what makes the short go away. When it does, the bulb will get dim or go out.
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Tuesday, October 21st, 2014 AT 8:31 PM