There is a tether in the tender. Read your instruction manual. You can attach an aux water tender to this via a tether in the tender. Select the AUX switch an the aux coupler fires and not your coal tender which would decouple the water tender. So I wonder if they got this wrong in the wiring that is preventing your rev light. You switch sound backwards.
MTH did this several way over the years with respect to how coupler fired, and how lights operated.
If you sent me just the tender, I could work through it and make sure it was wired right. Otherwise I would call MTH and get it fixed. Ask them to pay shipping both ways.
If john is correct an many have this wrong, they should know about it. G
