Is there an app that will allow me to play music from my iPhone (via Bluetooth) through my car’s speakers?

I don’t believe an app would suffice. I think it’s a hardware limitation. I could be wrong.

According to those smart-ass annoying iPhone commercials there’s an “app for everything.” But unless your car speakers have some sort of blue tooth receiver (and the app, in fact, exists), then you’ll need extra hardware.

Currently, I am able to make and receive calls wirelessly through my BMW’s speakers because the car has bluetooth integrated into it’s iDrive system for hands-free calling. However, the only way to get the music to play off my iPhone 3GS is to plug it into the AUX port.

Stereo Bluetooth transmission/reception requires the A2DP Bluetooth protocol, which is only in the 3GS iPhone; the Original “OG” iPhone and the 3G are monaural Bluetooth only, which works with car audio systems set up for hands-free telephone functions. Even though the 3GS now has A2DP, most car systems are NOT A2DP, to the best of my knowledge. I have not heard of any car audio adapters or aftermarket systems that have this feature.
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@Deepness: Does your BMW cable charge the 3GS? My MINI Cooper S (which is engineered by BMW) came with a cable that ran the controls off the steering column that also charged my OG iPhone, but I get a “Charging is not supported with this accessory” message with my 3GS… so I was just curious if yours works or if you need a new cable. I was told by my MINI/BMW Dealer that a compatible cable is not yet available, and the converter that Apple sold me was only for the 3G, not for the 3GS.