Which is it? Front or rear switch that blows the fuse? You need to isolate the switch that has the issue and follow it on.Disconnect the wires from that switch and hook up a Multimeter on "Continuity" to the connectors that you pulled the wires from.The switch should be open circuit(high resistance or high Ohm reading)across the connectors if it's good;and show a direct short(low resistance or low Ohm reading)when you operate it.If the Multimeter shows a reading of low resistance or low Ohms before you operate the switch then it has an internal short and will have to be junked.
Years back,i had a bike with a faulty brake light switch that would blow the main fuse and bring me to a stop.The problem was that the fairly simple switch had developed an internal short and every time the brake was applied the fuse went 'pop',and i came to a stop.
Of course,you could have wiring under the guard that has been attacked by the wheel--check your power wire to the rear tailight for damaged insulation or bare contact to metal..