As a stepping stone to my utlimate goal of a one cable iPod auto cable (power and audio), I made a Firewire<->Cigarette adaptor cable. (I hadn't seen any previoous instructions, so figured I'd take pics and document a bit)

1. Solder a wire from pin 1 of the Firewire connector to the +12V output of the auto adaptor
2. Solder a wire from pin 2 of the Firewire connector to the GND of the auto adaptor
3. Use your voltmeter/multimeter to check for shorts, opens, etc.
4. To be truly safe, use a lab bench power supply to power the cigarette connector and verify voltage output on Firewire pins.
5. Plug this in to your car (without the iPod). Use multimeter to verify that pin of the Firewire adaptor is giving +12V with the ground probe of the multimeter on pin 2.
6. Assuming your readings show you did this correctly, use the Apple Firewire<->Dock cable to connect to your Dock.
7. Plug your iPod in to the Dock. Viola, you should now be supplying power so that you can use your iPod in the car indefinitely.
8. Go back and tidy up any soldering, cover with electrical tape, etc.

I put this and some pics at http://home.nc.rr.com/zimmy/ipodsimple/

In a short test tonite, I charged from 426 to 486 while playing music (using the digital battery readout hack) over a ~40 minute drive.

Hope this info helps some other electronics hackers out there.

Hope to make the "ultimate" cable in a week or two and will post results.

Mike