Hello Bob,
As John says the sidelights are separately fed. In the 12N plug the Lh sidelight and number plate light are the Black wire and the Rh lights are the Brown wire. If you have a meter or test lamp, put between the White (Negative)and Brown, or White and Black to see if it's coming from the car.If it's OK at this point, you need to start looking at van fuses or bulbholders etc. Remember fuses blow for a reason!