You can find vehicle wiring diagram in the vehicle's workshop repair manual. Depending on the make, model, and year, owners can purchase this manual from Haynes, Chilton or the vehicle's manufacturer. These manuals are available at most automotive supply stores.
Ensure proper safety precautions before working with any vehicle electronics. These are also described in the workshop repair manuals. Newer models of vehicles often don't have a workshop repair manual available right away. Consult with the vehicle's manufacturer for detailed wiring diagrams. For older vehicles, manuals often contain information for numerous vehicle models and years. Ensure the wiring diagram matches the vehicle.