in my experience, this is not an answer because ALL 12volt chargers I tried introduced so much noise into the MP3/Ipod, that it was impossible to hear the music while plugged into the charger. I wanted badly to not have to charge the device in a motel room while on the road and wanted it charging to stay fully charged while riding AND enjoying th music.
the 110v home/wall charger plugged into a small inverter allowed the device to work perfectly on the road while charging, so I was motivated to wire up a setup that way.
I see your point about the electrical noise on the mp3 player. It is a valid point, but I solved that a different way. I use my Zumo to play mp3s. I was able to eliminate engine noise by grounding all my electronics (Zumo, Starcom, etc) to the same exact place. If you are running your mp3 player using the cigar lighter then you may want to try redoing the ground wire to that so that it is attached at the same point as everything else. Mounting grounds to a point on the frame is not the best way. Connecting directly to the battery is better. I use a Centech fuse panel which is connected directly to the battery, both pos and neg. On the positive side, between the battery and the Centech, is a relay and a noise filter. The noise filter just got rid of the last tiny bit of engine noise, almost all of the noise was fixed by solving the grounding issue. The cheap filters sold by stores like Best Buy did not work at all. I found a better quality one at an online CB shop.