It was for a different project, however I have done just what you are asking... albeit for the Intravee and alpine KCA-420i
Here is a copy of the write up I did:
I've managed to get this done.
Connected it up to my car and its all working as it should, tested with ipod and ipod touch (I don't have an iphone to test it with).
The whole process is reversible so you can revert the cradle and snap-in back to OEM.
The long and the short of it is that:-
The snap-in and cradle both need to be modded.
You need :-
The iphone snap-in,
The cradle,
1 x 30pin iPod dock extender lead,
Tools:
A soldering Iron,
A hot glue gun,
A small torx screw driver,
A small phillips screw driver,
You need to remove the gubbins from the cradle (easy), and the gubbins from the sap-in (also easy).
The snap-in's OEM male dock gets replaced by the male extender dock plug (once stripped down), the dock plug ends are identical and you need to move the metal shield from the OEM dock plug to the extender plug, it then is plug and play into the snap-in.
Stripped down Snap-in
Stripped down Cradle
OEM snap-in dock
Extender dock
Close up of extender now in OEM location, the antenna needs to be un-soldered and then it can go back together
Put back together
Gubbins removed from cradle
Pass the lead through the cradle hole (it fits), I also did a touch of hot glue to keep the extender lead fixed to the base of the cradle.
In car
Ipod touch with no charging, it sits on the home menu when connected. (and give the no charging message)
Display
I had a spare BT cradle and bought the ipod snap-in (cheap), I was bored and wanted to see if it was possible.
Adding the BT pair button and the antenna is do-able (if you don't want to make it reversible), but that's an easy fix.
I doubt I'll use it TBH.
HTH