Originally Posted by Tourist in MSN

Agree with Unterhausen, but trying to add some clarity here.

If your headlight is the one I think it is, the headlamp is grounded to the frame and it has the wires attached to the light.

I assume it is a Shimano hub. If so, the little plastic connector with the wires that attaches to the hub is in two parts, you have to take it apart, switch the wires, reassemble.

Try it first without the taillight to see if the headlight works by itself.

Then if it works, add the taillight. If adding the taillight makes both lights stop working, switch the taillight wires.

If this does not fix it, we need better description of what you have including brand and model hub.