small speakers for MP3 player to mount on Harley

[Nottoman]

I have a client I do tech work for that challenged me to find a small but powerful set of speakers (they would have to be about 3 inches square or even flat would be better) that could amplify an mp3 player tucked into the little cargo bag fastened above his handlebars on his Harley Davidson motorcycle. Ideas??
Brainstorm with me here. The mp3 player wouldn't be able to drive the speakers probably, so the speakers would have to be powered - battery? do bikes have cig lighters?
Thanks

[iwanterols]

you will need a headphone amplifier

i dont know about the speakers, i doubt anything small could overpower the loudness of a harley engine