Check the wiring loom in your chassis at the rear. Usually the wiring loom divides after the rear suspension into a few sections, right rear unit, left rear unit ,side lights, number plate light, trailer socket, etc. On modern vans each section is divided by plugs. The chances are that one of your plugs has water ingress and a pin has corroded resulting no connection. Disconnect one plug at a time and give each side a shot of electrical contact spray ( wd40 may work either) . Check your lights after each plug.
If you find a problem plug and if you want to seal it from moisture, use a silicone based spray, similar to, the stuff used to seal engine sparkplugs in the feista of the eighties.
Happy hunting,
Malcolm