You will probably have to follow the factory wiring. Someone on the forums did something similar to power a device in the cabin, but I don't remember where he took it through.

I would check the bulkhead just inside of one of the underhood fuse boxes and see if there is a place (grommet) that you can pass a new wire through.

The other option is like I said... tap the hot on the heated seat circuit. The device you are running is probably VERY low amperage anyway, so it won't be a big load. That's what I did for my inverter.

Hope that helps... if not... check through the service manual schematics, take a picture or two and ask again... I'll try and see where you could go through, but I'm at work right now.