Why is the trailer socket not working , even though everything is connected as per the installation manual?

It is possible that the battery earth terminal was not clamped as directed during initial installation. This means the control module was not able to register with the vehicle system. Remove the fuses for the trailer control module for around 5 minutes to reset the module.

In addition, the trailer socket lock must be checked. The socket contact insert must click audibly into the socket housing.