It will be a lot easier to figure out if you have the wiring diagrams. This is your road map where to test with a test light or volt meter. If you have a Ford v10 you can go on eBay or diymanuals.com and try to find the Ford dvd for your year chassis. The dvd will provid service manual and wiring diagrams for all ford vehicles made that year. After installation use the pull down menu and pick your vehicle chassis like E450. To see the wiring diagrams you have to install the svg viewer. Click on tools then adobe then win2k then svgviewer and allow installation. You can print the wiring diagram and it will also give you the color code for each wire which I found to be very helpful. Good luck.
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