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People have figured out how to hardware hack line level into the CD audio path. This requires playing a silent CD
Heh, I did this once, to hack a boom-box CD so that it's CD output became its aux input. Fortunately, I was able to use any old CD, not just a silent one. But I see your issue.
My concern wouldn't be for the length of time you could fit on the CD, it would be wondering what the largest MP3 file that its poor little stock stereo could handle. I'll bet it won't play a 700 meg MP3.
I think what you should do is ballpark the amount of time your car can drive continuously on the highway between gas station stops, make an ordinary silent wave file of that length (perhaps a bit longer), and encode that. See if it'll even play that.
(Logic behind this: You don't need a 900-hour-long wave file because, even at the longest, the car is gonna have to stop for gas at some point.)