4 cilnder swap

Hi everyone, I am swapping a 4 to a v8 and ran into a problem with the ecm harness, my car is a 89, and the harness I got is an 89(at least the guy says) there is a 3r plug on the harness right by the ecm inside the car and I am wonder what it is, the plug is not on my car, I found a picture on...