I need help to solve my melting switch problem
The load that is being switched in your AC is the clutch that engages the compressor, which is itself belt-driven by the engine, so isn't part of the electrical load. The clutch is held against the driving face of the pulley by a large electromagnet, and like all such devices, holds a significant amount of energy in the magnetic field, so there's is a diode connected in parallel, that allows the current to decay in a controlled fashion and not generate large arcs across the switch. 
  Below is a common arrangement for the AC clutch wiring, I assume your switch is taking the place of the relay. If the diode has failed or become disconnected, there's a good chance that significant damage is being done to the contacts every time you turn it off, even though the current is not high enough to cause overheating or fuse failure in a steady state - typically it will be a few amps. The arcing at the contacts will increase the contact resistance over time causing the switch to eventually melt.
  The diode is typically on the back of the compressor, or built into the wiring harness nearby, but could also be in the fusebox as a plug-in device.