This is how most people wire up their cars:
They typically neglect to ground the engine back to the frame.
The engine is somewhat insulated from the frame. There is weak conductance between the engine and frame. You may have some through the motor mounts, gearbox mounts, diff mounts, and axles.
When electrons go to the engine via spark plugs and other components, the electrons have no way to return back to the chassis.
After a while, you'll have a slight negative charge in the engine block due to the excess of electrons.
You'll also get a sligh positive charge in the frame due to electron deficiency.
This difference creates an electric potential, ie. voltage, between the block and the frame. At some point, the electrons will spark and jump from the engine block to the frame, kinda like lightning.
I've seen current jump the gap via things like the brake lines, turbo lines, or any other thing that touches the engine to the frame. When this happens, it will fry anything it touches, including braided lines, or even hard lines.
This is how you should ground your car:
It's pretty self-explanatory. Everything should go to a single grounding point on the chassis.
This is crucial if you're using sensitive aftermarket electronics that operate on a very small range of resistance. For stock electronics it's not so important because the stock stuff is not as sensitive.
Good luck. Ask me if you have any specific questions.