Ecu ywc500143...?

Anyone know what this is for and, more importantly, what accessory do I have to buy to plug into the spare socket that exists between the Yellow and Black plugs. I initially thought the space was for the tow bar electrics (but I found that socket when I looked harder).

Edit to say - The power of Google tells me it's to do with the Parking Sensors. OK fair enough - but what's the gap for??

Think the gap is for front parking sensors