Fog light problem - constantly on then constantly off!

Went to my Landy (300tdi) which has been standing for a couple of weeks and noticed that the rear fog light was on which seemed strange (have not used it recently). Noticed the switch was in the off position anyway when I checked and the fog light dash light was on as well, however when I flicked the switch on and off the light would not switch off - strange. Decided to fetch my tools to remove the bulb (to avoid running the battery down) but when I retuned the light was off. Now the light can't be switched on! Any ideas? What's going on here? Thanks.

Faulty switch or relay (if one is fitted).

There shouldn't be any power to the switch or light unless the headlights are on. Sounds like a short circuit behind the switch/dash.