All good stuff so far. I have a few things to add:

- Often times the local auto parts store, with a focus on replacing stock parts, doesn't carry the parts you need for modifying the electrical system, or has gaps in their inventory. Or you may be in a remote location where a visit to the auto parts store requires a long trip. There are several internet retailers who focus specifically on automotive wiring supplies. I use and recommend Del City - delcity.net. I have also had good luck with marine places like westmarine.com and jamestowndistributors.com

- A high quality ratchet-type crimp tool will produce far more reliable crimp connections than the typical pliers-like tools. If you shop around you can find a good one for around $75 or so.

- Ordinary automotive wire needs some sort of sleeving for protection. Most auto parts stores sell plastic split loom, which works OK, and some sell the older-style woven loom, which works better but is harder to install. The better places usually have it in back because it takes up space and is hard to keep neat, so you have to know to ask for it.

- There are sleeved types of wire, like duplex brake cable and marine-grade boat cable ($), that don't need loom and make sense for certain installations where appearance is important and you need a ground wire as well as hot, like auxiliary lights.