I did a similar thing when I converted all my dome lamps to run off the house battery in my homebuilt.
Luckily GM uses a switched ground for the door switches, etc., so it was just a matter of replacing the source of the +12v power.
I located the appropriate circuit in the fuse panel, and then used one of those "Add-A-Fuse" blocks, but flipped around backwards:
Normally you use these to add a 2nd fused circuit by taking power from the supply side of the bus in the fuse panel. One fuse passes through to the output leg, keeping the original circuit "hot". The other fuse connects to the wire protruding out, which you could use to power something else.
If you flip it around backwards, you can use the protruding wire as an input
, and supply power to the circuit in question. Obviously you need to leave the "passthrough" fuse out of the slot, so you don't connect the regular supply bus to your new input voltage source. Play around with a voltmeter and you'll be able to determine which slot is the passthrough and which is the "add on" circuit that connects to the pigtail.
The nice thing about this is that it's reversible - all you need to do is yank the block out and put the fuse back in the regular location and you'll restore normal operation (nice if you're having a problem with the house battery, or whatever.) And it leaves everything properly fused. Technically, you could do the same thing with an inline fuse and just crimp a spade terminal onto your input wire and jam the spade terminal into the circuit-side of the fuse terminal in your block.
You could just do the same with the fuse location for your power windows circuit. (Unless there's something else included on that group that you DON'T want powered by the house battery.)