power in * efficiency = power out, not knowing what the efficincy is I'll assume 5/6 to make the math come out ok, 100W out -> 120 watts in, at 12V that's 10 amps. that's one amp per 10W output, assuming a similar efficiency for a different inverter, pretty rough approximations here.So, a 400W inverter would be pulling about 40A on the 12V side. The existing inverter is run off a 15A fuse, so you got to assume that the wires would be rated at at least 20 amp, you want the fuse to blow long before the wires.
http://www.powerstream.com/Wire_Size.htm says 12gauge can carry 41 amps, I'd go with 10gauge rated for 55A or even bigger diameter wire. Currently the switch in the dash closes a relay to provide power to the inverter, that relay is the same part number as the one you messed with for the power windows mod. i don't know what it is rated, but I wouldn't trust it to survive 40A. The heater blower relay is rated at 40A, toyota used a much bigger relay for that circuit. I have no idea where I'd try to tie in an additional 40A circuitI don't know for sure, but if I was going to do it I wouldn;t go through that much trouble for less than 400W. You might be able to get a 150W one to run on the existing circuitry, but I wouldn't try it in my car.Not worth it to me to do a ragingfish mod with the custom melted wire harness and the extra 50W wouldn't be worth the trouble/risk.A 400W inverter only puts out 400W if the load draws it, if you hook up a 60W light to a 400W inverter, it puts out 60W, and probably draws on the order of 6A DC.For the output of the bigger inverter, I probably wouldn't bother trying to run it to the dash outlet, or if I did I might run that in parallel and use the thre prong outlets on that are probably already on the bigger inverter to power a bigger load.