The wire coming out of the bulkhead connector is the source coming FROM the key. So you want to "unload that" to take load off the switch. So break it there before it branches off. Use that bulkhead wire to key your relay, and use the "other end" out in the engine bay as the load
You can pull power off the start relay "big stud" put a big fuse or 20A or so breaker in that line.
Another thing you can do is to run the relay coil ground to a little switch in the car and it can act as anti--theft
