Citroen xsara lx 2001?

I have no reverse lights and all the bulbs are ok as they are the same bulb as the foglamp and I swapped them to check.
I have 13.5 volts to the reverse light holder but as soon as I put the bulb in the voltage drops to 20mV.
I have put a test across + and - and get 4.44 ohms. Obviously I have an earth fault but it's finding it. Has anybody else had this problem? I'm posting this in the hope it is something generic like the reverse switch or faulty relay.