Brucey wrote:if you look at a shimano generator you will see that one connection is silvery coloured and grounded (the locknut bears directly against it), and the other one is copper coloured and is the 'live' connection.
as shown on "this image from SheldonBrown"
If either lamp has a proper ground connection via its mountings (and many do...) then it is vitally important to observe the correct polarity when wiring the lights up.
Almost all dynamo lamps earth to the mounting point - they wouldn't work with single wire connections otherwise.
However, you can mount them using a non-conducting mount such as the B+M handlebar bracket. If you use such a mount, cable polarity doesn't matter, otherwise it does.