The switch is shorted electrically or there is a mechanical obstacle preventing the opening of the circuit by the switch. If you decide to install a switch on the power cord, remember that it has to be installed on the live wire of the power cord and not the neutral wire of the power cord. To check the origninal switch, just perform a continuity test on the said switch with power cord unplugged. If the continuity is always there whatever the switch position, then it is shorted.
Michel Tissot, January 2007