CITROËN DS 4 Handbook page 274

Attach the trailer to the towball.
Attach the cable on the trailer to the security
ring located on the carrier.
Lower the trailer harness socket carrier by
pulling down on the ring visible below the
bumper.
Remove the protective cover from the socket
and connect the trailer wiring harness.
Removing the towball
Disconnect the trailer harness from the socket
on the carrier.
Detach the cable on the trailer from the security
ring on the carrier.
Unhook the trailer from the towball.
Return the trailer harness connector carrier to
the folded position.
Insert the key into the lock.
Turn the key anti-clockwise to unlock the
towball fixing mechanism.