It's under the armrest in my '04 Cayenne S....

 

Did you buy your car new or used? It looks like someone installed an MP3 player.

 

Just bought it. If that's the case, you think I can adapt that t my iphone?

 

plug it in there and try!

 

It is definitely an aftermarket AUX input cable.
The previous owner probably installed an aftermarket fiber-optic MOST interface or a less expensive FM modulator.

I anticipate that you will be asking what to do to listen to your music player.

To listen to your music player:
Plug it in to this cable, turn it on and and start playing at medium volume.

1)Try the CD changer controls to find your music (if it has the fiber-optic MOST interface).
If this does not work, try #2.

2)Go to FM and scan the band until you find the dedicated frequency of the modulator with your music playing. Save it as preset 0 (easier to remember).

Hopefully the interface or modulator was not removed by the previous owner, and he just left the cable behind.

Good luck