I like to do the same when i wash the car - I get in, "start" the car, then roll the window all the way down and hop out. Of course, as soon as the door closes, the car shuts off, but since i have the window down, i can reach in and tap the screen to turn the car back on. Then, just reach in and pull the window up. Your music will still play.
The downside is when you want to change the song or the volume, you have to repeat the same routine.