That's a limitation of the deck, not the phone I'm afraid mate. Only the new range of HUs allow for "two-way" control, where you can control music from both the HU and the iPhone.
For example, the stereo that I really want:
JVC KD-AVX77 DVD Receiver
One of the features is "2-way control for iPod/iPhone". I hit a button on the display and I toggle between control on the HU or iPod/iPhone. Some models even allow simultaneous control, without even needing to hit a toggle button.
Pretty cool, but as you can see, that AVX77 aint cheap!