In my opinion, the best way to learn automotive electrical is to get in there and try to figure it out. I used to be very intimidated by car wiring and electrical systems, but after years of messing around with it, I now consider myself barely competent - something I'm quite proud of.
One of the things that I've learned along the way is that *Grounds Are Everything*. Land Cruisers (the ones I've worked on, at least) are ground switched, which means that the switch is located AFTER the load on a given circuit. So in many cases, the switch/relay/etc is actually opening a path to ground so that the device (headlight, wiper motor, etc.) gets power. If the ground is dirty/rusted, disconnected, or if the wire is broken (or if the main grounds are somehow compromised), there will be resistance in the circuit and nothing will work properly.
So, my suggestion would be to get a test light and decent quality multimeter, watch some youtube and have at it! Come back to the forum with any specific questions you may have. Good luck!