Buy a CD that teaches German. "Rush Hour German" by Berlitz and the "Learn in Your Car" series are two CDs specifically geared for people learning while they drive (see Resources).
Find a German-speaking co-commuter. There's nothing like learning from a native speaker who can actually respond to your questions and critique your speaking attempts. Put a notice on GoLoco or Craigslist to offer free commutes for German lessons.
Enroll in a German class or hire a German tutor. Have the teacher or tutor make a CD for you to use for practice in your car that is specifically geared for your level.
Buy a book like "Everything Learning German Book." Record yourself practicing the lessons, then play the CD in the car.
Once you have achieved a working fluency, listen to audio books on CD read in German while you drive.