"The first thing I did was finding a cheap tape deck adapter. I found one easily enough at www.allelectronics.com for $4.50. Opening the unit, I cut the headphone jack and measured out the exact distance I would need from the tape adapter to the iPod shuffle. A piece of plastic that I removed from the tape adapter was conveniently used to act as the "arm support". Branching off the arm is a small piece of plastic, which I cut to the size of the iPod shuffle.
The headphone jack was then soldered back together and placed such that an iPod could be snapped into place, with the piece of plastic from the arm supporting the iPod from below. The connection between the headphone plug and the iPod is pretty snug, and stays on while driving.
So there you have it. Once you've loaded up your iPod shuffle, playing it through your car is a simple snap-in affair. I just keep the iPod tape adapter in my tape player all the time. After all, who uses cassettes?"