Clivewatson,
The dim of the Radar display is located (for Airbus of course) on the button of the In/Off ND display (double button) on the left hand side of the Pilot (right for the F/o), whereas the radar Ctl panel is located on the pedestal, with no command for the dim function on the display at all.
I do have one time a captain reported to me that is WR was not working at all.... I just shown him how to adjust the brighness of WR ( hopfully F/o was working, means not dimmed). To be meditate .....