WARNING: This page is about a different car, the 2010 Porsche Cayenne, 2009 Porsche Cayenne, and 2008 Porsche Cayenne. However, it is still accessible from the selected car via links, so may be relevant.
Open the socket and insert it as far as it will go.
Fig 1: Installing Passenger's Socket
Courtesy of PORSCHE CARS NORTH AMERICA, INC.
Press in the connector until it is felt to engage.