How to Control an I-Beam Trolley with a PLC
I would look in the pendant to see how the cable connects to the switches.

If you've discarded the pendant, then look in the control unit, to see how the cable connects to it.  You may have to draw a wiring diagram of the control unit to figure out the function of the wires.  

If the pendant only has two switches, figuring this out should be trivial.  If more switches, there may be electronics and serial data communication which will be more difficult to reverse-engineer.