Thanks to Joe Snoy for an answer to this one. It was purely an electrical question, and no hint about it in the manual.
The board in the boiler effectively establishes a ground from track power for all front LED's. Thus the blue common lamp lead from the decoder is not used, and no seventh wire is needed.
Hal
I changed the MRC decoder to a Tsunami, but don't see how to get both the headlight and Mars light to work. The LED's each have a lead back to the tender, both their common lead apparently goes to a small PC board in the boiler which I can't reach easily.
Has anybody gotten them to work correctly without a seventh wire back to the tender?