Portable hard drive with audio out?

I'm looking for something that will help me sound check my studio mixes in the car.
I want to plug an mp3 player or hard drive into my computer, bounce a logic or protools mix straight out onto the hard drive, then take it in my car and plug it into my 1/8" input on my radio in my car.
I can do it with an ipod or iphone but i have to put the file into itunes and sync every time, which takes time, and I don't want to clutter up my phone/ipod with junk rough cuts of songs.

This is probably overkill, but I have a cigarette lighter unit that plays SD, USB, or 1/8" audio files to the radio. It was under $10, but I ordered it with other stuff for free shipping.

http://www.mcmelectronics.com/product/60-10010

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