This (ancient but still funny) page explains how to mount a melodica (those little keyboard wind instruments that dub musicians use) in your car. As you drive, a funnel catches the wind and pushes it through the melodica, which you can then play on your dashboard: "The volume of the Carlodica is regulated by the gas pedal which also propels the vehicle forward. The faster you drive the louder it gets. For fixed dynamics, set the cruise control around 70 MPH. A beautiful tremolo effect can be achieved by accelerating and slamming on the breaks repeatedly."
Alternatively, this (new, but less amusing) page explains how students at USC are building a real driving-based music interface, based on this idea: "Anecdotal evidence amongst musicians suggests that generating an expressive performance is very much like driving a car. Really? (Thanks to JB and Mikey)