Blowing headlight fuse. How to diagnose?

Just bought a third hand WR and am starting an extended tour on it. About 300 miles in I blew a headlight fuse, and replace it. maybe 400 miles later another one, and then blew a third after 20 minutes of riding. I borrowed a multi-meter and started testing for continuity from the fuse to the light and turn signal housing, but everything seems fine. Am I going to have to wait till it blows again and then test on the side of the road? In case it's not obvious, I'm bloody clueless. No obvious grounds. The previous owner had added in a line to charge his Garmin, and has some charging ports added in too. No idea where they are wired to.

Thanks for your help.

Sounds like an intermittent short; it's possible that the previous owner exposed a wire while installing the extra outlets. The only way to troubleshoot this is to carefully follow all the wires the PO installed, and notice if there is some frayed wire that could be making contact with the frame.