Be careful with this.
The ipad's speakers are too small to reproduce the low-end frequencies, but a car stereo can easily give you the exact opposite problem — too much bass.
If you can, I'd recommend in investing in at least a good pair of headphones that will give you as "flat" as possible of a frequency response. I recommend the AudioTechnica ATH-M50x if you can't afford or don't have an ideal room for speaker monitors.