In addition to Martlet's comprehensive information...
If you need to select a holder with built in dock, chances are it was made well before the iPhone specs were released. If the holder can accommodate a 30 or 60 Gb video iPod (aka generation 5.5) you should be golden as it has the same thickness and width as the iPhone.
I use the same concept adaptor for my 05 Trailblazer. I put a RCA to Headphone adaptor and ran it next to my center counsel. I had bought the Incase Charger for my Ipod Video, and plugged in that headphone jack to the line out on the charger. Now I plug in my iPhone, charge and listen to my music. I do get that "Item not made for iPhone" message and tell it to cancel for airplane mode. What is really cool is that when i receive a phone call it it works just like the iPhone headphones included, so it stops the ipod part and you can answer the phone.Depends on your car.
I have one that hooks up to the 6 disc changer port on the back of my factory Honda radio (I have the non-Bose radio in my Acura RSX, same as most base model Hondas). The "USA SPEC iPod® Interface".
It has two input connectors:
- a standard dock connector cable,
- RCA connectors which can be hooked up to a headphone to RCA cable. I have both hooked up.
It has a dip switch to toggle between two options:
- playlists named HONDA1* through HONDA6* controllable through the CD changer controls for discs 1-6 through the dock connector, or
- playlists named HONDA1* through HONDA5* controllable through the CD changer controls for discs 1-5 through the dock connector, and disc 6 plays anything on the RCA connectors.
I normally use the RCA to headphone connectors, since controlling an iPod through a factory radio's CD changer controls only allows 5 or 6 prenamed playlists, and is annoyingly difficult. But I have both cables wired so that when I need to, I can use the dock connector to charge the iPod/iPhone while playing it.
I know it rings through the speakers when I had the RCA connector cable in the headphone jack, I'm not sure I've tried it with the dock connector in use. I answer the phone with an old (the first model?) motorola bluetooth headset, and when I hand up, it goes from phone mode through the bluetooth headset to automatically going back to playing music through the headphones jack.
You'll get the "may not be compatible, use airplane mode?" when you connect via the dock connector, it works either way. I do turn off wi-fi, which seems to help bluetooth connections in the car. And I do hear the GSM interference on the speakers when it checks mail every 15 minutes over EDGE, but it's not very long or loud. GSM interference is much more noticeable on my friend's '99 Passat when using the cassette adapter from the Apple store.
If your radio has a headphone input, I'd use that and a ipod charger. Or if you have true iPod integration like some newest luxury cars (my brother's 2005 Cooper Mini basically had a BMW brand version of the USA Spec I have, not quite full integration since it used 6 playlists only too).
Otherwise, I'd say a disc changer converter is the best you can do.
If you can't do that, a converter that attaches directly between the antenna cable and the radio is best.
After that, you're down to a cassette adapter if you have a really old factory radio.
And if all else fails, you'll have to spring for an FM transmitter.
But you'll find better discussions of your options on car audio forums.