An actuator mechanism for preconditioning a gearshift mechanism for movement into a selected gear such as low, reverse, or park. The actuator mechanism is in cartridge form so as to fit a wide variety of gearshift knob designs and sizes and the mechanism is arranged such that the pushbutton plunger assembly for preconditioning the transmission is moved to its retracted position by the spring of the actuator mechanism and is maintained in its extended position by the shift link of the shift lever.