Don't splice into anything that's in a wiring bundle, a lot of stuff is run by data signals and adding something to a circuit can cause changes in impedences, etc., that can really mess things up.
Best bet is to do like you did, run a separate wire back to the fusebox. Look in the owners manual and see if there are any studs in the underhood fuse box that are designated for ignition-switched accessories. If not, you can wire to the stud post by the alternator that GM provides for connecting accessories. Don't forget to add an inline fuse.