Once I had extended the y-cable by 1 metre so I can screen mount my iPhone it failed to charge quite often and charged very slowly when it did succeed, but it never disconnected from the data/music. I just used some decent brand new USB and Audio extension cables.
To resolve this I bought the following cable and used a standard 12V USB charger in that socket in the armrest to give extra juice for the iPhone.
Phone now charges reliably and works fine with the Y-Cable and extensions. It's still much slower than a mains charger but manages to keep the phone charging a little bit even when running TomTom.