I have a few older Soubitez bottle dynamos that work fine without a ground wire (in fact, they only have one terminal) and one modern Spanninga unit similar to yours that I ran a dual line for a ground wire. However, I think a ground wire is unnecessary on a steel frame. Try sticking both bare wires into the positive terminal. If both wires are too large, you may have to splice the wire before connecting to the dynamo (so that a single wire enters the push-in terminal). Good luck. That is a great looking fender mounted headlight. By the way, you can usually find chrome Soubitez dynamos to match your bike's chrome accents on ebay for a reasonable price.