69 camaro

i have a 1990 240sx. It had a sohc in it, i purchased a dohc from a 1993. i also bought a dohc wire harness from a 1993 240sx,everything is plugged up except i am having trouble with the plug that comes off the ecu on the harness that plugs under the dash by the ecu in the car.the new wire harness plug wont fit so i have to splice in the old sohc plug on the end, but the wire harness has 12 wires and the plug that fits into the car has 13 wires,so i need to no how to wire it in.
thanks,Micah