Spend as little as possible. If you are connecting through the headphone jack any 3,5mm M to M cable will do. You won't notice any difference between a 90 cent cable and a $9 cable.
EDIT: Same goes for using the iPod's line out. Cheapest Apple connector to 3.5mm possible. Keep in mind you aren't using anything close to an "audiophile" style player, so any sound quality loss that could be caused by the cable will have been caused by your iPod (and your car speakers if they aren't aftermarket) ten times over.
Remember, the car is probably the worst place in the world to listen to music.
I understand where you're coming from though. I drive a lot and so listen to a lot of music that way. My car has a decent stereo, but I would defy anyone to tell the difference between aux cables in a doubleblind test.
@coonana: Just one question: what "thing" do you have in your car? Because most decent CD receiver (manufactured in the last 15 years) have two auxiliary ports, one analog, and one digital (used mostly for CD changer, but in the last years it's also used for other sources). You might be able to get a digital interface for iDevice that will make the sound "perfect".
@DanceDanceKennypants: What the hell are you talking about? There's no better place in the world to listen to music! Also, it's what produces the best music in the world: the throaty growl of a V8.
@coonana: I've heard that the line out (the propritory Apple connector) can sound better, but I doubt the difference is profound. iPods just aren't good sounding music players (and I say that with the caveat that I can't live without mine), so I would still go for the cheap cable.
@coonana: Yeah, but built-in, at the back (you have to slide the radio out to reach it), you have two sets of ports. One's analog, it's nice because it's very easy to use, but I'd say the only use for it would be if you want to use it for a CB radio... The other one's digital, used mostly for CD changers on modern cars (that's probably the case on the Nitro), and there are some other thi9ngs you can plug to it, like those interfaces to have iPods connection. What makes it far better than the analog one (beyond the obvious quality improvement) is that it can transmit many more things than audio signal. And I believe Ford uses that port for their Sync (and believe me it sounds nice!). Anyway, it's not really easy to find, most of the time it's brand specific, but it's the only one that's worth the investment (in my opinion). Here's one example, in a Renault Megane, this one in particular sucks the big one, but at least it allows to have a digital like between the iPod and the car radio, so it keeps a good quality (obviously it's the black box in the back).
Edit: Thinking of that, I'm wondering if this isn't a specificity of non american cars, given that american are always on a different standard. I know that the 300C didn't have the standard things, but the Nitro is much more recent... If you know how to do that, just pull the radio out (watch out for the cable, don't pull to hard) and check to see what ports you have available. The only way for the best possible sound with your radio is through digital interface anyway.