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I use a GPS and Wayfinder with my N800 quite a bit and really like it. However the sound, though much, much better on the N800 than any other hand-held, is not as loud as I'd like it.

Currently I'm plugging the N800 into my car stereo and that works very well, except I can't listen to the car's radio.

I'd like to get a set of mini-speakers, somewhat like the Nokia MD-4. Problem is that the Nokia MD-4 runs off 4 AAA batteries.

Does anyone know of similar car mini-speakers that can run off the car's 12V cigarette plug rather than off batteries; or perhaps has a mini-USB plug as the power input for the speakers?
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I have some compaq-branded USB-powered speakers. An ebay search for "USB speakers" turns up lots of matches. You could hack together a cable to get power from mini-USB, assuming it's not over 100 mW, but I bet they are.

Better solution is something like this which takes the 12v from the lighter socket and changes it to 5V USB. They also work great for charging ipods and other devices. I use something similar to power USB speakers occasionally for the kids so I can listen to radio up front and they can have better sound in back from the portable DVD player.