Can you connect your iPhone up to car stereo systems via bluetooth to play music..? I was sure that wasn't possible.
However, I had a similar problem with my first GT and got the dealership to install an iPod cable for me, it cost less than £100 and just ran an ipod connection into my glove box. It acted as if it were a CD changer - it took the first 10 playlists from my iPod and treated them as each CD in the changer menu, so you could control the iPod from the steering wheel, flick between CDs to change playlists, forward/backward to change tracks.
It worked really well. Only problem was that Apple have changed which pin in the connection feeds charge into their iPods (and the iPhones) now, which meant my iPhone still worked, but complained about not being able to charge. It only took a £15 'thing' from the Apple store (I forget what it was called) that plugged into the cable and then into the iPhone to change that back.
I only don't still use it because I bought another GT and haven't gotten around to getting another one installed yet..
Veloces, for what it's worth, are excellent (and local to you). Give them a bell, I'm sure they'll be able to install one for you - if nothing else, my old GT is still in there somewhere, with the iPod lead still installed...!