Several members have asked for a 'wiring basics' thread to remove the 'magic' that seems to attach to it, so here goes... I'll do this in several parts as there's a LOT of material to cover. I'll try to dispel some myths and misconceptions, and just general misinformation that's out there. First, my bona fides; I'm a retired electrician with over 30 years in the industry at multiple levels. I've designed systems and taught electrical theory, so while I won't claim encyclopedic knowledge, I know more than enough to talk about automotive wiring. Now, the first objection I'll hear is 'but that kind of wiring isn't the same as car wiring!'. Yes, that's correct, but electricity doesn't know where it's working, a car, house, etc, the basic theory is the same no matter what. AC or DC, no difference.... The OEM wiring/harness that came in your vehicle is engineered. The factory has full data on all connected loads, uses many purpose-built components, and knows to a large degree just how much many of these loads will be used at any given time. They do make compromises in the design many times, mostly for cost reasons, and occasionally get it wrong. But by and large, a factory-designed wiring system is an extremely reliable part of a car and rarely needs repair during the 'normal' life of the vehicle. But over time, wire insulation deteriorates and switches fail, the system gets altered, and all the other calamities you can run into, to where a replacement can be easier than repair. Add in major modifications to the vehicle, and a new system can become nearly mandatory. If you're starting with a vehicle older than about the early 60s, the wiring used in the older vehicles will be failing in multiple ways by now unless the vehicle has been very well preserved. So, you need new wiring. Now, there's multiple vendors offering 'replacement' harnesses, with many claiming how easy theirs are to install. But at best, these are only semi-engineered; they've used 'typical' values for circuit sizes that may or may not be correct for your particular install, and worse yet, some ignore what engineering data is out there when sizing circuits and related parts. Sometimes this is just ignorance, sometimes it's for cost savings, allowing them to limit inventory and reduce their costs to build the harness. What I'm going to do is attempt to spell out (in simple language) what you need to think about when doing wiring. While a pre-built harness may be just what you need, I'll try to give you the knowledge to make an informed decision and/or be able to ask the right questions to your vendor (and know if he knows what he's talking about! LOL). If you're wiring a very 'basic' vehicle (lights, horn, minimum creature comforts) this can be pretty simple. But if you're upgrading with power accessories, AC, big stereo, etc, it can get complicated. I'm going to be conservative, so that if you follow my recommendations you should end up with a wiring system equal to or better than OEM for years of reliable, safe service.