you should not assume that a black wire is ground, in this case it is correct but it not always the case.
it is not the switch that is causing your short. the switch supplies ground to the green/yellow wire to activate the taillight relay. concentrate on the green/white wires, these wires come after the 15A fuse. check the harnesses at the fuse box under the dash.
the kick panel is the location next to your left foot on the driver side and right foot on the passenger side. the rocker panel is what is next to the seats.
have you had any work done recently? it doesn't have to be electrical. any kind of work.