Basic Car Audio website has hundreds of pages about car audio and electronics. Some of our favorite pages are:

OfSoundMind.com has several tutorials about basic electronics including wiring info, power distribution and more.

Getting Started.

Ohms Law.

Series Wiring.

Parallel Wiring.

Formula application.

High End equipment.

Show preparation.

Power Distribution.

John Adams Basic Electronics website.

Theory. Gain the basic understanding of electronic principles including Ohm's Law, Circuit Theory, etc.

Practice. Putting the theory to work.  This includes sections on how to solder, multimeters, and many projects.

Components. Learn about various electronics components.

Forums. Post your basic electronics related questions here for others to answer and read.

Reference Data. Resistor color code info, plenty of calculators, chart, electronics data and more.

Wire Guage Calculator from xtant.