636 tail light problem

hi kids, get your thinking caps on
the tail light of my '03 6 doesn't work when the bike is on (whether the engine is running or not), however the tail light and number plate light DO work when the key is in the park position - therefore no busted bulbs, and nothing wrong with the wiring to the lights. Rear indicators work under all conditions, brake lights do not work at all (but they did before this quirky little occurence)
I would have thought the system would all run off one circuit, so I don't think it's the fuses (but I haven't checked that yet - we couldn't find the fuse box when we were messing with it - I'd left the manual at home
) The problem is completely reproducable, and I literally watched the lights blink off all of a sudden (with the bike on but not running) when the problem first occurred.
Any ideas/thoughts would be really appreciated because I get the feeling that it's something really simple that I can fix myself, but the nice boy at the workshop said "yeah it might take a few hours ($$$) to pin down what's causing it..." cheers for the help kids
-J.

I would have thought the system would all run off one circuit, so I don't think it's the fuses (but I haven't checked that yet - we couldn't find the fuse box when we were messing with it - I'd left the manual at home

Any ideas/thoughts would be really appreciated because I get the feeling that it's something really simple that I can fix myself, but the nice boy at the workshop said "yeah it might take a few hours ($$$) to pin down what's causing it..." cheers for the help kids
