JDM N374 ECU Harnes 3rd Plug Question?
 

I just dropped the motor in my car for the swap. I just found several problems I didn't count on. It is an 86(s4) and I have an S5 T2 Engine. The dash Hanes plud is different, and has 24 wires instead of 22(I need the plug as well for thew (22), but the problem I really need addressed is, what is the 3rd Plug on the Engine harness that fit through the wall to the ECU. 2 Plugs are for the ECU, but I don't know what the 3rd is for. Also, where can I get wiring schematics or the pin reading(more importantly) for the dash to ECU Plug(24-S4 & 22-S5), and what do I do about the 3rd plug. I appreciate all the help I can get. Thanking you in advance.-Blake