Did you get a harness with or without the relay connector?
You can probably leave it as it is and then you might need to do something with the wires under the seat if that isn't done already. Check that there is 12v on the starter relay, yellow wire, when you turn the key to start.
If not then the yellow from the red plug in the seat belt relay is the yellow to use in the starter relay. I believe this will be the case. Please check that this yellow comes directly from the firewall connector, and has the 12v like above. You will probably need to move this wire to the starter relay, or jumper it to there.
I just modified mine a few days ago. I made my own harness to relocate the starter relay, delete the seatbelt relay and move the battery to the trunk. But I didnt check to closely before I took it apart, to see what you would do like in your case where you wanted to keep the original harness.