I think I need more info to really advise you... I wouldn't use the factory power or ground wires if the Alpine V12 is rated at much over 100 watts. There is a formula you should adhere to, although I don't have the information on it, see a professional for advise at any high end car audio store for more details, but it's something like this... For Y watts of rated amp power, you need X type of power wire gauge. The more the power, the thicker (lower numerical number) gauge wire you need for the power wire. I would ballpark this wire between 2 and 8 gauge depending on the load you intend to put on it. You likely want to go directly from the battery to the amp with a fuse close to the battery, say within a foot or so. If you need to run more than one amp, add all the load up, and bring back one heavy duty wire all the way to the back and then use a power distribution panel ( it looks like a T) to break it out (attach all the devices to this - think of it in terms of you home circuit breaker box, one set of power and grounds in and power being distributed to multiple devices). I recommend you install another set of fuses just after the "T" for each of the devices to insure electrical isolation from one another. You could add a supercap in front of the "T" to keep the voltage up at 14.4 volts or individual supercaps after the "T" in line with each amp as well. That would be your choice. Remember, the higher the voltage, up to a certain point, the higher the amps power - but also the more power demand and drain on the battery I would think. Again see a professional for advise and installation if you're unfamiliar with any of this after you do your research. I would also advise re-reading the Alpine manual that came with the amp when you installed it in your F-body. Alpine is great about the technical stuff. See their website and email them with any questions as well, and they have a FAQ section under the support section on ther North American website at,
http://www.alpine-usa.com/
From the Alpine you want to add a power detection wire that would connect to your head unit that will turn on the amp when you turn on your head unit. This is a very low current voltage wire, I would think 18 gauge or so wire would be fine. What you can use your OEM power and ground wire from would be to power a CD changer, XM radio receiver, or other low power consumption peripheral equipment such as that. Be sure to use the power detection wire again so that when the device is not being used, it's not drawing power.
With regards to your speaker wire, if your have good speaker connections and no intermittent static, meaning you may have either bad connections or chaffed wire somewhere in the car, you can try to use the OEM speaker wire harness, unless your pushing some big ineffecient speakers which require more than say, 40 watts or so, per channel RMS to run. If you run into any problems, try soldering the wires onto the speakers first, if that doesn't work though, you'll have to rewire the speaker wires.
Did I answer all your questions? If not, email back Jim