...via the mp3 port inside a car? And how does it work?

PS: specifically for an iPhone.

Ty.

If the Head Unit has a USB port, then use the USB to whatever the connector on the Iphone/MP3/Phone cable that you use to charge it.

Same with a MP3 player that uses a Mini or Micro USB connection, just get the proper sized USB connector on the other end (all 3 USB formats use the same pinouts or wiring set up, they just differ in size)

If it is one of those mini ports, like a 3.5mm jack, that is essentially a Headphone Jack and you can use any headphone cable that has the proper ends on it. In this case, the player controls the volume and track selection, rather than the head unit.

As was mentioned, if your car has a USB port then use a USB cable like you'd use to plug it into a computer. Most cars support iPhone/iPod for this. With non-Apple players the car will most likely have to support the USB Mass Storage profile and have the ability to search a storage device for music. Some car systems will do this and others will not. My car will support this, but only up to a 2GB device, so the feature isn't terribly useful for me. I use Bluetooth anyway.

If it's just an AUX audio input, you most likely would use something like this:

http://www.monoprice.com/Product?c_id=102&cp_id=10218&cs_id=1021802&p_id=644&seq=1&format=2

...to connect the headphone jack on the phone to the AUX-in in the car.

Thanks!