There's an aspect of most motor vehicle circuits that tends to confuse people. These circuits appear to have only one connection rather than two between the source and load. Simply put, you can follow a wire from the source to the load but that seems to be the end of it.
Compare this diagram with the Basic Electrical Circuit diagram I posted previously. The second (bottom) connection is missing, replaced with a pair of lines ending in symbols like this:
Motor vehicle manufacturers use the metal bodies and chassis for the apparently missing second connection, in a way that is generally called a common ground. This is represented in circuit diagrams with ground symbols. Whenever you encounter a ground symbol, interpret it to mean the source or load is connected to a common ground.
If you add a plus sign (+) above the source black box and a minus sign (-) below the source box, you have an accurate diagram for all motor vehicle circuits.
(This applies to modern motor vehicles using a negative ground system ... very old motor vehicles sometimes used a positive ground system that turn out to be less efficient and more problematic.)
