Just find a head unit with AUX input or add optional AUX input to your existing head unit (usually connected through CD Changer control) and connect it to your PDA / Cellphone headphone jack. For cellphone you can use the optional wireless car kit. Now you can hear the sound through your stereo system. If you want separate system from the stereo system, just install a portable speaker on the dash, connected to your gadget. Oh yes, another easy option, use a bluetooth handsfree, hear them directly in your ears when you need them, without everybody in the car hearing it.