If you only have an RD you probably don't need a Junction B if you can run a wire all the way to the front junction (Junc A).
On the other hand, you might need to add a battery into the mix somewhere...
I always forget something
But if he had the 2-port shifters the battery could also be wired direct.
@hippy started
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On the other hand, you might need to add a battery into the mix somewhere...