There isnt any specific Iphone adapters for the car yet. But there Ipod adapters you can try, and some other options.
Depending on your car radio, if you have a CD changer/AUX input that isnt being used you can buy adapters to use the input for your Iphone. They make some for specific radios to Ipods or RCA inputs.
You can try the Ipod adapter and see if it works, or get one with regular RCA inputs and use your headphone output for audio to the adapter. You would need a headphone to RCA cable to plug from your Iphone to the adapter.
They have 2 types of FM modulators, one is inline/wired and the other is wireless. Wireless will 9 times out of 10 have alot of static. Wired will be a clearer but may still have some static at times. The wired version will also have RCA inputs, so you will need the headphone to RCA cable for it also.
You can get either of these at a local Circuit City or Best Buy. They use the inline/wired FM mods for mobile video and Sat Radio. And also carry the Radio specific AUX inputs for the same thing.
Here are some examples:
Ipod to Chrysler radio
http://www.circuitcity.com/ssm/Metra-Chrysler-Compatible-iPod-Adapter-CRIP01/sem /rpsm/oid/119805/catOid/-15887/rpem/ccd/productDetail.do
Wired FM mod with RCA inputs
http://www.circuitcity.com/ssm/Audiovox-FM-Modulator-FMM100A/sem/rpsm/oid/80974/ rpem/ccd/productDetail.do
They also have adpaters for JVC, Pioneer, and Alpine aftermarket radios.
Hope this helps!