I have just tried it with the LED unit hanging out and all the wires stretched out and it seems fine now. All I have to do is ride with the guts of the tailight hanging down pointing at the ground and it'll be fine!
Seriously though, it looks like there is a bullet connector in there that is not very well shrouded and it must be almost touching one of the exposed terminals on the light so the vibration does the rest. The Casoli light unit I have plugs into the original light fitting so there was no rewiring to do, but there is not much room in there.
More likely, the problem developed last year when I fitted the number plate tidy because that required a rewire of the plate light. I was obviously a bit careless in my routing of the wiring.
[Edited on 4-5-2005 by HW]