I have an AE92 AFM GZE also. I'm pretty sure the plug/socket you are referring to is for the front/rear window water solenoid (the grey thing). It is meant to sit in-line with the washer motor and the front squirters. When you switch on the rear wiper jet, it is supposed to turn on the water motor (in the little tank) while diverting the water to the rear from this point.
FYI, I had trouble getting the solenoid going (turned on the front ones and water would dribble out the back one) so I disconnected/bypassed it. It looks like yours has been bypassed also (so only the front water jets work).