per here, starting at page 338:
http://www.ft86club.com/files/BRZwiringi.pdf
both trigger wires(positive and negative) off the relay go into different magic black boxes
*my initial impression is that the system is switched negative, with a ign-controlled feed, but the black boxes don't allow me to confirm that easily. if that were true, you would require both a constant power into the relay trigger, as well as adding wire to switch the ground leading away from the relay.
so i would suggest running an external relay and switch setup directly off the battery-- my initial impression/assumption of the existing system operation would require 50% of that work anyways.