Wiring diagrams are very useful tools to track down problems or help rewire your car. A diagram will show you how the wires are connected in your car. If your lights quit working, you will be able to track down the wire(s) that is causing your problem. Unfortunately, some cars specific diagrams do not show features that may have come with the car. A lot of times this happens with tachometers and radio/speaker packages. I guess the Big Three did not think anyone would care about these things 40 years later! If you are buying a diagram for your car you may want to check to see how complete it is.

• You should be receiving information for the complete basic car: (including the exterior lights, engine compartment, dash & instrument panel, steering column, interior lights, horns, etc).
• It should be easy to read and follow.
• It should be made for your specific car. In some case things stay the same from year to year, but it most cases there are significant changes.

The old books available from the manufacturers were all created in black and white with small print and arcane codes. Alot of these have been reproduced into wiring booklets or condensed into manuals that cover several cars. Some car manufacturers were more detailed then others when documenting their information. Mopar (Chrysler Dodge Plymouth) included the most complete information, GM the easiest to read (not as complete) and Ford spread theirs out by system (ignition & charging, interior & exterior lights, instrumentation, etc) which makes for some seriously interesting diagrams. AMC and most of the other small American manufacturers weren't too bad in both the layout and detail.
If you want to figure out your wiring with a superior diagram ClassicCarWiring  has taken the old factory information and applied new technology to make your life easier! Everything is in color (the same colors as the wires in your car). They are easy to read, and they are laminated to boot!