Check out the schematics below (click to enlarge). Regardless of the model year or sunroof option, you've gotta get access to either the TAN 755 circuit coming right from the Rear Integration Module (RIM) or the DARK BLUE 75 circuit coming off of the RAP relay (if you have it). Either circuit should be able to drive the added I coil talked about above.
IB's suggestion: if you've got the RAP relay already there with your sunroof, find a way to tap into the 75 circuit. That way, it's already fused and you don't worry about putting any unnecessary load on the RIM 755 output. If you don't have a RAP relay (no sunroof), add one and if your sub takes less than 30A, use that as the power feed.
If you're concerned about the feeds to the fuse block, I don't think you should be. There are 2 - 10 gage wires feeding that block each capable of 80A at around 75 degrees F. The wires get derated for current as temperature increases and when interior temps reach about 170F, the current rating decreases to 63A. So, the one wire that supplies the pusher fan feed, also supplies the power seat and lumbar motors, reverse lights, trunk release, various modules, blah, blah, blah. Those loads would nominally be around 5A and will approach 80A IF (big IF) all the seat motors are stalled (i.e. both passenger and driver seat switches are pressed and the motors run to their max position in any direction).
Since I don't think you'll be listening to the stereo AND running the seats to stall, I doubt you'd have any wire gage problems. The only downside I can see would be minor. The sub feed would essentially share the same feed as the seats, so unless you put in an adequate power supply with good input filtering, you might hear a pop every time you move a seat motor. Inrush current from a motor will put a lot of noise on that feed.
thanks IB