I noticed that no one has responded to this yet, so figured I would. I have done this exact thing on a couple of MTH engines that had similar problems, worked great. Completely eliminated the dead spot.
Refer to the attached PS-2 wiring sketch. PS-3 is similar. The red circled area is where the power is supplied to the board. This is a diesel diagram, so it shows two inputs, one from each truck. Steam is identical, but will have only one set of power wires coming from the tether harness. The new tender pickup is wired into the other two pins on the 7 pin plug, which you should find have no connection presently. Observe polarity carefully using this sketch.
For the roller parts you will need to contact MTH parts (Midge) and she should be able to set you up with a roller assembly that is compatible with your tender trucks. From an operational point of view it doesn't really matter which truck you attach the new pickup to, but its easier to avoid the one that the speaker is mounted over. Use the other one if you can.
Cheers,
Rod
