This is assuming you have a jailbroken iPhone and Summerboard installed.

1. Connect your iPhone to your 30 pin adapter.
2. Allow your car CD player to start accessing music form your iPhone
3. Open Summerboard
4. Change the position of the Theme Status Bar. So if it was on, turn it off, and vice versa.
5. Press your home button.
6. Slide to unlock.
7. Open your iPod.
8. Enjoy

This was done an a 1.1.2 jailbroken iPhone and a Pioneer deck with a 30 pin adapter

I will be testing more combos tomorrow at best buy with all their demo models.