Useful Links/Resources

WinISD is a useful program for calculating and simulating different box designs, and seeing how your subwoofer responds to box changes.

Basic Car Audio Electronics(BCAE1)
BCAE has alot of useful general electrical knowledge, more detailer car-audio related knowledge, and a lot of small tools/calculators.

Simulate "Cabin Gain" in WinISD
Interesting post on simulating/accounting for cabin gain when calculating box designs using WinISD

Crutchfield Subwoofer Wiring Diagrams
Crutchfield has an excellect collection of images on how to wire your subwoofer(s) for the different impedances(ohms)