[Canadian Psycho]

Are the wires connected correctly? Specifically between the trigger switch assembly and the motor.

Are the motor connectors to the proper polarity?

Provided the gearbox is internally good it would be wiring issues. The rapid heating/melting of wires is due to overload, either there is a short some where, or a wire is on the wrong location causing the motor to attempt to turn in reverse, hence, the jam and overload.