Originally Posted by DiegoFrogs
That makes a lot more sense. Probably the part of the circuit that detects the functionality of the taillight is on the ground part of the circuit rather than the "hot" part, and the current reduction through that part of the circuit tells it something is wrong. It's sounds more like a design robustness issue rather than the cost of the circuit itself.
It's a moot point for me, because I have a different headlight and a plastic fender, but my technical side was curious.
It doesn't strike me as necessarily an issue with design robustness, rather that the new headlamps are designed with features that require certain things of the other elements of the system, i.e. the taillight current shall return through, let's call it Terminal A. If this requires two-wire power distribution that's something the dealers and users must be made aware of. In my worlds of automotive, aircraft, and aerospace electronics such constraints are quite common. No component specification is complete without definition of the system interfaces, based on electrical, logical, and mechanical considerations. It's a "this bike is no longer a toy" moment.
The upside is, "you can't just put on the light any way you like" means you get some more features in the lighting system. The downside is, you can't just put on the light any way you like. I suppose in a sense robustness has been traded away to achieve more function.
I plan to go with these lights on both my bike and that of Mrs. Road Fan, so I'm thinking about modularizing the wiring so I can actually remove a fork, cockpit, or fender as a subassembly without having to cut off spade lugs - try to engineer our bikes for electrical serviceability.