Ok, so here are what each wire in the subwoofer harness is:
-purple/green (-) negative
-green/white (+) positive
-purple/red: sub amp turn on
I hooked everything up to a new 8" pioneer sub and rockford fosgate amp 300/1 and it all works great. I would say I have about 3x the bass performance, maybe more, than what the stock setup has. I'm probably not a very good judge in sound quality, but I know this is much better. It's not nearly what I had in my 2004 F250 with a 12" sub powered by a 1000 watt amp, but it's exactly what I was looking for. It's a mild upgrade replacing what is already there without having to run power lines and new grounds. I also ordered the amp remote gain control that rockford fosgate offers and mounted it below the front dash.
I have some new component 2ohm speakers for all 4 on the way that I will put in next weekend and will be dynamatting the doors. I'm just trying to improve from stock until I can plan out (and afford) a full-out high end system. Although, I may not do the high-end plan since this is already starting to sound much better.
Here are the sub and amps that I used: