Sorry, more info: Shimano dynamo hub, running a B&M LUMOTECH IQ Fly. So the taillight is run off contacts on the back of the headlight unit. The Toplight doesn't ground to the frame, so it's a double wire running from the headlight. Ground v. current is clearly marked on the headlight, but no marks anywhere on the tail.
Heard back from the mfg:
"Dear Mike,
if you connect the "TOPLIGHT Flat plus" to the dynamo or the headlight it is alike which contact is current and which earth.
Best regards"
Great...
What say ya'll--if I'm looking at the back of the unit on the headlight and the ground is on the left, current on the right, based on this response, I should assume the same on the tail light--ground, left; current, right, looking at the back of the unit? I mean, it's couldn't possibly be something silly like: on the headlight, ground is on the left side of the bike and so it should be on the left side of the bike at the tail light, which would actually be on the right as you're looking at the back of the tail light...?