Are you talking the 12v current activation switch on the 12volt control /display panel.?.
Check power in fuse/s I've know one to make and break then blow altogether.
Remove your c/van battery check the cells are topped to correct level, unless it's a sealed one. A test to see if it's to do with the leisure battery is to put a car battery in place and try switch again. If symptoms are teh same then it has to be a wiring/joint,fuse fault. So afraid then it's 'Check the cable,fuses and joints time, from battery through to swiitch, also ensure ther is a good earth on the negative lead of the battery as well as a bad earth can cause problems like that. How many cars have you seen with dim side llight ? if they cleaned the lights 'earth' it would come back bright. possibly the postivie side as well I know I've done it.
Good luck elec fault are buggers.
Bob
June 2009