I need a little more information. Is the Motor AC or DC? What voltage is the motor. How much Amps does the motor draw?
The easiest way to do this is with a MOSFET and Relay. But the type of relay depends on the requirements of the motor. I you do not want to open the switch box. you will most likely need a Relay with a coil voltage of 5-12 volts DC and a contact rating depending what country your in. If you in the USA 110 - 120 volts AC with about 15 amp continuous. There is an example for a MOSFET controlled relay on the wiring web site. Get me some more info and I'll help you get it rolling.
By the way could let the forum know what project you are working on? It make it more interesting for everyone.