Fuse keeps blowing

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 twelve 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 is 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.

Turn the heater fan off because it is a very high-current circuit and will cause the test bulb to be bright even when there is nothing shorted. If it appears the fan circuit is related to whatever is shorted, you will have better results using a head lamp bulb instead of a brake light bulb. A brake light bulb allows only one amp of current to flow. A low-beam head lamp bulb will pass close to five amps.