You can find accessory power with a cheap connectivity tester (the pointy screwdriver looking thing with a light inside and a wire with an alligator clip). If you get a non-LED style one, it'll be easier.
Turn the key to accessory, clip the test light to a good ground, and start poking wires until you light up. Bingo, you just found a lead that's hot under accessory. Now, you'll probably want to figure out what wire that is, just so you're not robbing power from something sensitive about it, hahaha.