Dynamo-powered rear lights are typically designed to fit either the end of the mudguard or the end of the rack, which should have a mount for such lights. Even old racks should have a lamp mount, though at some stage the hole spacing was standardised. Rack mounting is by far and away the best way of mounting a rear light, but if you have neither rack nor mudguards, you'll need to fettle your own mounting bracket. Take no notice of the + and - signs on the back of this lamp. All bicycle dynamo systems are intrinsically AC, so positive and negative has no meaning. These connectors could be swapped around and the light would still work.
