I just looked at the wiring diagram for a 300EX and the same wire on it is black/yellow also. It runs from the ignition control module to the positive on the ignition coil just like I thought. Any time you want to hook a kill switch on any engine, just hook it to the positive on the coil and when it's grounded, it will kill the engine.
Most 4-wheeler problems are caused by a loose nut connecting the handlebars and the seat!!
You only need two tools in life -- WD-40 and duct tape. If it doesn't move and should, use the WD-40. If it shouldn't move and does, use the duct tape.