specifically, I want to know what guage wires I should use for a 400 Watt (800 peak) converter that I am going to mount in the rear cargo area of my FJ60. Also, what Amp fuse should I run? I will connect it directly to the battery.
Search "inverter" in this section and you should find lots of info and some links to wire gauge charts. Fuse w/in ~8-12 inches of the battery, then run wire to the inverter in back.