Useful Links/Resources
WinISD
http://www.linearteam.org/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)
http://www.bcae1.com/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
http://www.caraudioclassifieds.org/forum/tutorials-quick-references/3081-%5Bcomplex%5D-possible-simulate-cabin-gain-using-winisd.htmlInteresting post on simulating/accounting for cabin gain when calculating box designs using WinISD
Crutchfield Subwoofer Wiring Diagrams
https://www.crutchfield.com/S-mNgPl7pq9fx/learn/learningcenter/car/subwoofers_wiring.htmlCrutchfield has an excellect collection of images on how to wire your subwoofer(s) for the different impedances(ohms) https://www.crutchfield.com/S-mNgPl7pq9fx/learn/learningcenter/car/subwoofers_wiring.html