Actually I would not do it that way. I'd add a relay, OK, but cut the blue "run" wire coming out of the bulkhead. Use the wire from the bulkhead for the trigger, and use the relay to feed all the loads the key "did" feed. If there is a voltage drop problem, it affects everything under the hood fed by that line.
Also a good idea to put a fuse link or breaker for the supply line.......something Ma did not do!!