You will need to run at least
an 8 gauge wire off the battery to power the amplifier depending on how much power you want! In your case since you will be using an old factory type head unit, to get the sound to the amplifier you will need to splice off the factory radio output wire that goes to one of the speakers into a line level input on the amplifier. Most newer amps have line level inputs built in with a crossover! Line level inputs may also be called speaker level inputs.
In other words:
Conventionally when you hook up an amp, you get the audio feed from the radio to the amp by using an RCA wire.
Most amps now also allow connection using line level inputs with a built in crossover or "Xover", which means it can take the signal designed for a speaker and use that instead of an RCA. This is so you don't have to upgrade your head unit to one with RCA outputs.
If you have an amp already and it doesn't have the line level inputs, or has line level but doesn't have a built in crossover you will need an external line level converter or an external crossover, respectively. You will still splice off the factory speaker wire to get the feed, only the external converter makes the level into an RCA type connector to allow conventional hook up though RCAs. In this case you will need an external crossover as well. They are what separate the frequencies in the music, making it so your subwoofer wont try to play high pitched singing notes in the most basic terms.
If you need more clarification let me know and I will try my best.
EDIT: Here is a video I found for basic how to
Red 1988 GT under restoration!
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[This message has been edited by Shho13 (edited 01-24-2015).]