Sounds like your clutch might be going on the tranny. Wiring harnesses are pretty straight forward as long as it’s labeled well. I usually do all my own wiring as purchasing a harness can get pricey. I would ensure you have all the right connections on your new harness prior to installing it. Just do a one to one comparison and make sure they jive. You could also do a resistance check on all the connections which may identify a break somewhere before everything is installed. The extra 10 minutes can save you a lot of time and frustration down the road.