Ignition fuse

A trick to narrowing this down is to replace the blown fuse with a light bulb. Insert a pair of spade terminals in place of the fuse, then use a pair of small jumper wires to connect those to a brake light bulb. The bulb will limit current flow to a safe value of around one amp. When the circuit is turned on and the short is present, the bulb will be full brightness and hot so don't let it sit on carpeting or against a plastic door panel. Now you can unplug things and move wiring harnesses around to see what makes the short go away. When it does the bulb will get dim or go out.