Hello everyone!
Sorry for my bad English,I use a translator...
Finally I managed to charge my Iphone 3GS connected to Pioneer F900BT.
I have thoroughly studied the problem and I discovered that the 3GS performs an additional check on the terminal's USB connector.
The Usb protocol provides 4 connections:
Pin 1: +5 volts
Pin 2: D -
Pin 3: D +
Pin 4: GND
Match the connector Iphone :
Pin 23 : +5 volts
Pin 25 : D -
Pin 27 : D +
Pin 16 : GND
If you want that your charger (in this case our Avic) functions properly, you must be sure that the pin D-and D + respectively, there are 2.8 volts and 2.0 volts.
If the voltage is lower,the iphone display the message "accessory invalid .."
AVIC not provide these voltages correctly.
How to solve the problem?
The adapters that you can find in internet ( like Scosche Passport and other similar ) may not work because they are designed to reduce the voltage from 12 volts to 5 volts (firewire).
In our cable avic do not have the 12 volts to pin 19/20 of the dock connector, therefore, these adapters can never work.
Do not spend your money.
I thought of 2 solutions.
You could use the voltage of 5 volts present in the USB connector to get the 2 missing tensions, using the following scheme:
http://meletta.net/wp-content/uploads/2 ... schema.jpg
Resistors can be solded within the 30-pin connector that we use to connect our iPhone, but I have not tested to open it, to see if there is sufficient space.
Alternatively, as already written by other members, you can use a cable "Y" USB to connect simultaneously Avic + iphone + 12 to 5volt cigarette adapter.
I am using this system and it works perfectly.
This system works because the cigarette adapter is connected in parallel and provides the missing 2 tensions
Obviously you have to buy a cigarette adapter compatible with iPhone 3GS otherwise, the problem returns! 
)
Even the Y-USB cable are not all equal, I have recovered 2 "Y" cable from external hard disks, but only 1 works.
Enjoy your Iphone !
Valerio from Italy
