When you are wiring, you should run a heavy gauge wire from the power source (battery?) to the amp. You want to avoid having that power line run alongside speaker or signal wires, like a CB antenna, as it may cause interference on those signal wires. If you do have to cross signal wires, do so at 90 degrees to minimize interference? What size wire?
http://www.gtsparkplugs.com/WireSizeCalc.html
Your ground wire should be close to the amp itself and should also be a heavy gauge wire. There are existing ground points back by where the Monsoon amp would be, underneath the gray plastic on the side in the cargo area.
As for running the power wire, it's through the firewall as already pointed out. Remove the plastic door sills by pulling up (they snap in place) and run the wire under the carpet or along the sills. Really you just figure out ways to tuck it out of view as you get to the back seat, etc.
Be sure to seal the hole in the firewall with RTV silicone when you are done to avoid water intrusion. If you drill a new hole, be sure to use a grommet to protect the edges of the wire, and then silicone to seal it.
For example, this is how I ran my CB wires. While you see a ground wire in this pic I've since rewired it and only the power wire goes through the firewall. I used a ground closer to the CB itself.