If you want to test the chassis as is, straighten a small staple out and see if the end will push into the plug, if it will bend the staple in half and push it into holes 1 and 2, repeat with another staple linking holes 3 & 4. Make sure they don't touch each other. Your loco should now run. If you're happy with this and want to do the job as you envisaged, remove the sticky tape between the wheels in your picture, the 2 wires there are from the motor, cut them there and pull the ends connected to the plug out of the hole. The 2 wires left still connected to the plug are from the wheels, cut them so you can shove them up through the hole and connect one to each motor wire. Swap them over if it runs backwards.