Planning your audio build doesn't mean "do I want 2 12s or a 15 inch subwoofer?". Start with what's already installed in the vehicle, really analyze what's hurting the most. In most stock vehicles a subwoofer IS the place to start. It adds in the missing frequencies your midrange speakers just can't produce. Then, depending on budget, you can start to look at speakers, 4 channel amplifiers to power them, and possible electrical upgrades which are NECESSARY after a certain point to prevent equipment damage. If YOU have a project you're working on and need assistance, or don't know where to start, let me know how I can help!