Volvo V40 - Rear Parking Lights Not Working

Grateful for any help with the following as this is really puzzling me
and has just failed its MOT because of the rear lights.

With the ignition off and the parking lights switched on (middle
position on light switch) only the front lights work, the rear lights
don't. The rear lights themselves are OK as switching the ignition on
causes the rear lights to light correctly.

Have checked all fuses, swapped the light switch unit but still not
working. Managed to identify the blue/yellow wire from the light switch
unit which using a multimeter is working correctly however I cannot
trace where this wire goes to on the fusebox. The Haynes manual says
the wire goes to C2 which should then connect to G10 which is a grey
wire to the rear lights. Does anyone know if the connectors are marked
as G or C ?

Grateful for any responses.

Alan