Hi Dave
The rear socket should be between the rear air vents in front of the middle passenger seat. The one on the boot should be on the driverside boot panel. Bizarrely not all a3 came with the extra sockets as I think it depends on the model and wether your car has the luggage pack installed.
But i believe the wiring should be there for them and they can easily be fitted :)
Hope IV helped
Cheers
Steve