Replacing both leads is always a safe option, especially if electrical work freaks you out. Electrical wires themselves do not generally just simply go bad...just the end/termination connections or in some cases the connection point where the wire is crimped onto the actual connector. I hate crimped connections! The main ground wire is pretty easy to find as it is located forward of the rear tire on the engine block near the starter. You can trace the path of your old wires and locate the connection point. I saw a picture of someone's engine ground connection here on the forum. I wish I could find that picture again as and it was absolutely filthy. The area is prone to dirt and moisture because of its location. If you ride in the rain often, live near the ocean or salt air environment, ridden your bike on salty or dirty roads, this is most likely the problem area. When troubleshooting, a good inspection of things is a good place to start. You would be surprised at how many things can be fixed with just a good look if you know what your looking for. Always work from the most simple solution to the more complex.