If it is anything like the two-truck version, there should be a screw in the stack that will remove the upper portion of the boiler.
once that is removed, there will be a screw in front of the cab, and one more in the water-filler hatch on the fuel bunker. Once those are removed the cab will come off. tHat should expose the motor and wires.
If you need more, I might still have a copy of the old Rivarossi exploded diagram (again, for a two-truck Heisler).
