Originally Posted by
znomit
If its one light running off the dynamo it shouldn't matter. If it doesn't work try the other way.
If it was critical there would be a warning.
I'm not sure about this. I don't have personal experience with the B&M system, but it appears that the taillights connect to a special connector on the headlight rather than the dynamo, and it may be that this connection is rectified/regulated. Sorry I can't be more helpful without more knowledge of the system, but I just thought I'd inject a note of caution.