Just acquired a fairly new (2021) MTH Premiere NKP Berkshire. Under the tender are the two select switches - Pardon my ignorance here but what is the upper one selecting please?
This my 1st PS3 steamer...
To simplify- inside this tender, there is a tether that can be connected to another trailing aux tender used on long excursion trips. That tether again is currently zip tied up inside the tender and requires you to open the tender, cut the zip tie, and then route that tether out of the frame near the rear coupler to use with a MATCHING PS3 version- aux tender.
The switch either connects the rear coupler, and rear reverse light of this tender (Coal tender setting), OR in (Aux) it sends those over the tether to the aux tender being pulled behind this one so the rear coupler on it and it's reverse light are controlled.
For you- in your situation- leave in Coal mode for this tender.