I've just installed the factory subwoofer in my ST170 using my Sony CDX - GT420U headunit as the 9006 I got with it has issues with loading cd's.
The wiring was already installed from front to rear (which I understand is true with all ST170's) so I just modded the RCA sub output and amp remote from the Sony to suit the Ford wiring. Loads of people will tell you it can't be done, but it can - you do need a HU with a sub output though.
The standard wiring is already connected to a constant positive feed and ground, but it's not a heavy high ampage feed cable and may not suit the particular amp you're going to use. You'll also find that on the boot end of the sub loom the plug has two red wires (+), two black wires (-) and three white wires with a red stripe - one of white/reds is the + signal input, one is the - or screen input, and the other is the amp remote. Nice of Mr Ford to make that confusing ?????

If you're fitting a complete aftermarket system I'd run all new cables to be on the safe side, but if you can get hold of a factory ST subwoofer, I can recommend that very highly - very stealth, and excellent sounds.

If anyone needs the wiring info for connecting the Factory sub to an aftermarket headunit, I'm happy to share my knowledge.