I'm pretty sure the starter wiring circuit has a fuse inline, under the left side fairing in a little box, along with another few fuses that control lights etc. I'd check that first (if you haven't already), but if you can hotwire it at the relay I'd suspect a dodgy starter button. Also, check all the wiring behind the gauges is connected properly, where the starter button connects back onto the main loom. You're definitely on the right track though, get an ohm-meter (digital multimeter) and test for excessive resistance through the starter button by disconnecting the plugs from the loom and putting the ohm-meter across the two wires, with the button pressed. If it's high or open circuit you've found the problem. Good luck!