Most every alternator has 4 wires coming out of it. I don't have the wiring diagram in front of me but what ddgonzal said sounds about right. I disconnected and took out the stock voltage regulator and just hooked up the remaining alternator wires to the A15 alternator in the same fashion that the A12 alternator was wired. I did this by just extending the A12 wires to the other side of the engine bay.
I just wired up my SR20's alternator and the basics are still the same, one wire goes to the dash warning/charge light, one to the ground, one to the 12v+ constant and one to the 12v switched I think....like I said I don't have the wiring diagram with me right now so take that with a grain of salt.
Anyways the way I had my A15 wired up it always charged fine, only when I started it up it required me to rev it above ~3500 rpm for the charging to kick in....must have something to do with the Internal Voltage Regulator, cause as soon as you revved it to that point for the first time since startup, the lights would all get brighter....whatever, it worked for almost 10 years.
The SR20's alternator supposedly gets up to 90 amps!! Without having A/C, P/W, and just tons of electronics, I should have a pretty healthy charging system with it!

Good luck!