Using line out could also help save you some battery power since w/ headphone out you'd need it cranked probably at least 3/4 of the way up. Either way will work w/o any damage to either the JB3 or the car stereo. No matter which way you do it, make sure the car stereo volume is turned all the way down, they slowly increase the volume to your liking. Dont want to blast out a car speaker with loud clapping or whistling at full volume on both JB3 and car stereo.
Jim