1995 Nissan Altima meter fuse

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.

For intermittent problems like yours the bulb may be dim already. Watch what takes place when it gets bright. That's when the short is occurring. It could be due to the rocking of the engine when you shift between reverse and drive. It could be due to the body flexing when you drive over bumps in the road. The bulb limits current to a safe value when the short occurs, in this case about one amp.

What's the easy way to replace the engine wire harness?

You get the new one and start on one end by unplugging a connector and transferring the plug to the new harness. If you have to pass it through small openings where there isn't room for the old one and the new one, you can use tape next to each plug and write a description of what it connects to or you can draw a picture of each plug with a few wire colors and locations for reference, and write those on a piece of paper.

Replacing a harness is severe overkill for a short. The only time I ever replaced one was when the old one was badly damaged in a crash. If your troubleshooting points to a problem with a wire in a harness, look for a damaged area, especially where it's rubbing on a sharp edge of a metal bracket or on the body, or where it passes close to hot exhaust parts.