One thing to consider when hard wiring an inverter is that the further away from the power source you go the less juice you can get out of the batteries. Here is the problem that I ran into.

In the Columbia and Century, the battery box is approximately 18 feet away (after routing cable), and if I was going to use 1 gauge wire, the resistance was so high that I would drop the voltage by 5.8% @ 120Amps (don't remember the actual voltage, but it is close). So, if I was going to run my 1500 for that length of a run, I would have to run 00 cable to compensate for the voltage loss, and it would give me somewhere around 3.8% @ 120Amps for voltage loss. For laughs, I plugged in the same amount using 250MCM cable, and the drop was 2.6%@ 120Amps. So, no matter what you do, you're going to have voltage issues.

If you are using your CIG plug, then you want to get your inverter as close as you can to the batteries (like the C-Pillar on the truck). The inverter will be able to pull more juice and won't be as likely to cut out on you. HOWEVER, it my kill your inverter down the road!

Here is a link to the site I used.

http://www.nooutage.com/vdrop.htm

There are other sites that I have used that will give you better ideas of what to use.