Radio Shack sells a cable that's perfect, and works great with the iPhone 3G. If you have a case on your iPhone for protection, you might have to open up the headphone plug opening (on the case) slightly, or shave a tiny bit of rubber off the plug, but mine works great!

Here's the one I use! For $7.49, I don't think you can beat it!!
6-Ft. Shielded Cable, 1/8" Stereo Plug to 1/8" Stereo Plug - RadioShack.com
One end plugs into the jack inside your glovebox........the other end into the headphone jack on your iPhone. I also found a really nice Caddy for my iPhone at ShopKo for about $15. It attaches to the heater vent and holds your iPhone (or other mp3 player) perfectly. You can even access the bottom plug for hooking up a 12vdc charger.
The company is called Bracketron, Inc.
www.bracketron.com
The part number is PHV-200-PB.I believe the model is the PH-Grip-iT, and it comes with various mounting options!
smark.....you might want to take a look at this if you don't have any mounting options. It has a button that very quickly opens the "arms" holding your iPhone to release it, and when you put it back, you simply squeeze the arms back together to hold it in place!!