That's not a plug. it's so the certain signals can be sent to multible places. I found 2 behind the speedo in my car, and that had 1 speed sensor in, and 3 out (ATTESSA, ECU, HICAS). Dunno why yours is at the ECU, maybee there was something like a Super AFC, and when they removed it, they put that in to pass the signals back to the ECU.