Specs:
http://www.cardomain.com/item/ZAPS100X (10 sec. too late...)
I'd say all you could use is 8GA. However, if you've seen those Wal-Mart 5GA kits (not 4GA anymore, and I think the 8GA is now 9GA), they have the larger wire run back to the trunk, then you have a splitter with two smaller wires. So, you could just use the splitter as a reducer on those wires. Then the other one you could use to power anything else you want to do, such as lighting for your trunk. If you don't have a split seat, it might be nice to build a wall with the sub on one side and your amp on the other, have some lighting on it. Such as they did on Unique Autosports (Unique Whips on Speed) and have the amp sit mounted on a piece of plexiglass that's been frosted (sandblasted) with a couple neons behind it to light it up. If you have a bunch of stuff in your trunk a lot of times, this would be a good thing to do. Cover it, put some fans on it that are ported through your rear dash. Fairly simple.
But like I said, if you do that, you could put in a power inverter with a relay to turn it on. Mount it in the trunk with some wire extensions to the front, and put in an AC outlet and you can just plug your AC stuff in and flip a switch for it to work. Good to have a fan on this too, and have the wires protected heavily due to the AC power. Wouldn't want that frying anything. But that's something I'd like to do with my inverter. I do have a 9-way splitter under my front carpet...
http://ecoustics.epinions.com/Zapco_100W_Mono_Amplifier_100X__Two_Channel_Amplifiers_100X/linkin_id_~3035301/display_~reviews
You want some real good info on the Zapco, check out:
www.carstereos.com
I did a search and brought up 10 threads that included it. You have a whole bunch of audiophiles on there who aren't as headset to go Rockford and Sony (bwahaha). They'd rather go with the powerful unknown stuff that doesn't say 5000wts. for $50... Of course, they might have fun and put something like that in a fishing boat or Festiva or something...