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This is not the first one that is out but seems to be the best so far.
If you can end up using a voltage regulator, it requires 12V this would be great for everyone to add this with a external hard drive for example a wd passport will work but not sure how well it will work with the bumping of the road I would suggest a ssd hard drive or a flash drive will work to do this as well
This thing will let you play any file format and and good variety of music formats also. The only down side is you will not have volume on the remote or be able to use touch screen to change the song you would have to use the remote to change songs and DSP Processor Drc controller for volume.
In reality this is best for movies and shows or if you like to have a big collection of lossless then you could do that also.
Lets also not forget you can also do it this way... You can have a screen with navi and then use optical out on the media player then get volume controls and do it that way as well. Something like a bit one or a dsp6 will work cause it has its own drc controls for volume the possibilities are end less!
The only thing you would have to do is get a processor with a drc that controls volume and this should be a great sucess
Newegg.com - ASUS HD Media Player O!Play HDP-R1 HDMI Interface - TV Tuners & Video Devices