Turn the ignition switch to "unlock", shift to "neutral", THEN start the engine and shift to drive. If the fuse blows, it's due to the engine rocking. Look for a wiring harness that has a wire that's rubbed through the insulation and shorting. If the fuse doesn't blow, shift to "reverse". If the fuse blows, there's a short on the wire going to the backup lights and the fuse is blowing in the "reverse" position as you're shifting from "park" to "drive. Another clue is the fuse will still blow when shifting to "reverse" without the engine running.

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.