When you buy replacement wire harnesses, ask how the harness was assembled. It is frequently assembled with peanut oil or palm oil as the lubricant to slide the wires into the loom. This attracts mice like crazy. If you can find a harness that was not assembled with edible oils, you are better off.
Once it is too late for that and the harness is already installed, you have to deter them by overriding the smell of the edible oils:
Rodents don't like Pine Sol. They also don't like hot peppers and moth balls. I dissolved dehydrated powdered hot peppers of various sorts from the dollar store and poured it into 50/50 Pinesol water mix in a 5 gallon bucket. I knew I went overboard with the peppers when I got done mixing: I took the nitrile gloves off, washed my hands-- and then later on touched my eye-- and it about swole shut on that side. Phew! Weeks later I found a few dead rodents. They had ripped their eye balls out and shat themselves liberally. I guess that repelled them alright... Since rodents clean themselves with their mouths when they get into smells they don't like-- they end up ingesting that mix and getting it in their face/eyes as well as getting the shits. They are then very unhappy critters at that point... I would paint the mix on the tractor frame where the wires are so they get a good dose of pain (as long as you don't have cats/dogs that could get into it). I would also paint it around the bottom of the shed, let the wood soak it in.
Also, when you re-roof the shed, consider using translucent plastic/fiberglass roofing sheets that lets light through and then keep the hood up on the tractor when in the shed-- rodents don't like being in daylight in the open. Makes them feel vulnerable.
Last, consider making a 5 gallon bucket mouse trap loaded with something like peanut butter. They can already smell the peanut oil on the harness... so consider it a lure to get them into a trap instead of your tractor.
