http://www.clubknowledge.com/Car_Audio_FAQ/forum - There are some very experienced installers and techs at this site. They don't sell any products, so there's no incentive for them to recommend one brand over another, other than quality/value/personal experience. It's a well moderated site that doesn't allow flaming.
http://www.bcae1.com -This site is an excellent resource for 12 volt car audio. It's more like an online electronics/car audio course, but you can go to just the section you need information about, and it's written so that each section can stand on it's own.