The automatic seatbelts move forward when you open the door whether the key is in or not, they just don't go back until the key is turned.
I've done the same thing (rx-8 seats in a vert). I used a add-a-fuse tapping the fuse block directly to the seats.... used a 15 or 20 amp fuse I think. The add-a-fuse says 10A, but its not continuous so as long as the fuse doesn't blow in the 1-2 sec that power is requested I'm good.
