I have a Nakamichi CD-40z deck in my car and I connected the RCA-to-mini cables to the AUX-INPUT in the back.

I've tried hooking up a Muvo TX but was extremely disappointed by the sound quality and volume. I tried messing around with everything on the Muvo including the volume and EQ but had no luck. I have also read that the Ipod Shuffle gave similar disappointing results. The thing that confuses me is the claim that the Shuffle sounds better than all the other Ipods. Yet, I just read another post on this forum where someone hooked up a 10gb Ipod in their Honda using the aux-in rca cable setup and said it sounded great (also confirmed by 2 other people on that post)

So here are my main questions:
Will I get this same result with all flash-based digital music players when connecting it to a car stereo? Also, does the quality of the cable that I use to connect the mp3 player to the aux-input play a huge role in affecting SQ (radio shack vs. acoustic research from Best Buy)?

The main players I am currently considering are the Iaudio U2 and I5 and the Ipod Mini 4gb. And if you know of any other mp3 players that sound good in this car setup, please let me know.

What you probably need to check most is what voltage the player can put out.

I think a lot of car equipment uses a higher voltage for "line level" compared to home equipment to give better S/N ratio with all the electrical hash in a car environment.
At the same time, a lot of small players put out a relatively low voltage to the phone jack because they expect you will use high sensitivity ear buds and the whole player may be running off just a 1.2 volt battery (single AA NiMH or slightly run down alkaline)

edit: There is variation for sure. My Neuros spec is 30 mw/channel (unspecified impedance) compared to Iriver iFP 190 at 10 mw/channel (16 ohm phones) So the Iriver would be roughly 1.1 volts peak to peak or 0.4 volts rms.

From my experiences with multiple mp3 players, finally getting an iPod, none of them had good quality when going through the headphone jack. If you connect your mp3 player, flash or not, and it does not have a line-level output on the mp3 player then the quality will not be good. I don't have much in my car but my CD deck has a aux input in the back and I have upgraded the speakers. My iPod has the best quality out of them all as I purchased a car adaper and line-out kit. Some iRiver mp3 players have built-in line-outs which have good quality.

Bottom line, use a line-out connection that is either built into the mp3 player or comes as an extra attachement.

Are there any hard drive based in-dash units yet? I don't mean a PC crammed into the dashboard. I keep looking and waiting.

I think I've finally decided:

Ipod Mini 4gb (silver or pink...since my friend has green)
Sik Din

How's that? If anyone has any input before I pick it up feel free to chime in. Thanks for all your help!

Can't you just use a portable headphone amp for line out?

my friend got a $50 tape deck from radioshack (cant find it on thier site) that had a built in line-in mini port on the front. It had good, not great, quality and worked well with low power devices like MP3 players.