Multi-step plan:
1. Hardwire iPod to cars stereo, bypassing it's native (the stereo's) abilities. Have a wireless card installed to iPod (it is a novel). Operate iPod remotely.
2. You can go four ways here (at least):
a. broadcast the worst music you can possibly think of, trying to annoy the driver to distraction, following him/her until they exit the car in disgust.
b. play a fake radio broadcat, giving "news" of fake traffic conditions, thereby directing your victim to a spot where you can take their car, or some variation of.
c. If the iPod was coupled with a mic and a record feature, you may be able to record sensitive information about codes and other pertinent information. May work better with an armored car or something.
d. Use iPod (with operating system) to hack and bypass modern cars onboard computer. Stall car. Steal car either by posing as tow-truck (can use option c to help with this (when and where) or by driving off when person goes to get help. with this latter option, you might trick the car to show that it is out of gas, so you have a few minutes while the person walks to the service station.
all pretty unlikely, but there you go...perhaps someone can build off of these.