I just finished replacing my blown factory subwoofer with the Pyramid WX65X sub from Parts Express, and the thing sounds great! I was a bit skeptical as I used the factory enclosure and amp.
The cone of my factory unit was torn all around the edge. The job took me about 2 hours (including dragging tools out, removing old console, trimming to fit, rewiring, coffee, pee break, more coffee, chasing the puppy around, cleaning up residue from soda previous owner spilled, and packing up tools).
I have a 2002 Jeep Wrangler with a Pioneer stereo and factory speakers. The sub sounds great! It impressed me, especially since I only invested $27 including shipping!
Now note that I am not into radio competitions and such, but for my purposes the thing sounds great (it will shake your behind)! I highly suggest this mod if you are tired of hearing the fuzz from the blown factory sub, and don't want to waste $400 for the factory unit, or more on system mods!
Oh yeah, for my '02, the brown wire was positive, and the gray wire was negative. I hope this helps someone out there!