The bulb contacts in the light are bendy copper. This loses its springiness in time. Cleaning and bending them a bit may help otherwise you might have toreplace the light fitting.
However, check with a multi-meter if there is 12 volts at the light contact. If not check the 12n socket and plug.
So one of the lamps on the NS is not working - what about the other(s) on that side? If working then the fault is within the vans wiring as they have a common power supply.
Lamps often corrode in their holders - was there any? If so check the inside of the holder and remove any deposits there to ensure a good ground contact for the body of the bulb. Spray the metalwork of the lamp holder with WD40 to keep corrosion at bay
I would check the 12v fuse, multimeter check the car socket, check pins in the 12n plug, clean the bulb holder and bulb. The lighting problems I've occaisionally had on my van have all been resolved by cleaning the bulb and bulb holder.