What more specific do you want to design?
Making wiring is one thing, designing logic with relays another, and designing electronic circuits with active components yet another thing. There are many levels of complexity, but the best way to learn is to try things. Obviously you will fail at some, but every failure means experience.
I think what most people benefit the most from is studying other peoples' designs.
The best open automotive electronic design I know of is MegaSquirt - studing that will also learn you some facts about software and electronic fuel injection - if that's in your interest area.
http://www.megasquirt.info/