You aren't thinking like a programmer :D
Write a program that uses the last.fm api and then keep a record of the songs you listened to while driving. When you get to the computer fire up the program and enter in the songs you listened to.
Personally I think the whole idea of sharing what you listened to is a big meh.
But does anyone care what someone else is listening to? I can't understand the point of updating your profile with the current song that's playing, other than for a marketing reason to promote those songs...