Take a look at the service manual (which is stickied on ic.net). Specifically:
-Look at the connector physical document
-Turn to page 174 "Under-dash Fuse/Relay Box"
-Find "Connector C", which is "Option Connector D, Hot in ACC & ON"
-Note "Connector C" isn't fused on the underdash fusebox, but is fused in the engine fusebox.
-Open insight schematic.
-Turn to page 10-1 "Power Distribution - Under-hood Fuse/Relay Box"
-Note "Option Connector D" is connected to a 30 A fuse (#16) in the Underhood fusebox.
-Note fuse #16 is also used by 8 other subsystems, so there's zero chance you'll be able to pull 30 A through it.
-Note also that running a fuse at more than half its rated limit will cause it to fail much faster than you'd expect. Most manufacturers rate fuse current as "time to fail after four hours at continuous load." However, cutting the time-to-fail exponentially decays with decreasing current; below 50% rated current, the lifetime is in the "thousands-of-hours-to-failure" range.
...Back to the drawing board:
-Look at underhood fuse #17 (40 A), whose only load is the power windows.
-Find a creative way to tap into the lead from from engine Fuse #17 (WHT/BLK) to underdash fusebox.
-At 75% continuous load, a typical fuse is going to last a few thousand hours. You almost certainly won't place a constant 30 A on the fuse, so you're good to go.
-Make sure you lower inverter demand before operating the windows, or risk blowing the fuse.
...Another option:
Tap directly into the DCDC inverter, which connects to the 12 volt battery via fuse #2 (80A). As an added perk, power sourced while driving will come directly from the DCDC converter without first passing through the battery lead. Note the car won't be able to measure the load sourced in this way.