As anyone who knows me will tell you, I'm no genius, but it sounds like you've got a faulty ground. Aviators, water and electricity all take the path of least resistance. Have you tried turning on the lights, getting off your bike and then touching the bar? If the fuse remains intact, you probably are shorting the thing out when on your bike, by providing a less resistant conduit for the juice. BTW you wouldn't necessarily get shocked.
