There's a few different ways to connect it , 1 like the last guy said through the aux port, 2 through the a lead the connects to the factory CD player so that your system thinks you iPad is the CD player in the boot , 3 using a Bluetooth module so no wiring is needed except to charge it .
Means you have use of all your iTunes music and also great to watch videos , lots of different radio apps and also brilliant sat nav apps I personally use a Tom Tom app which I brought a few years ago and is so good I use it on all my iPads and iPhones ,
If anyone is interested in having one made for any iPad I now make a kit which I can fabricate into the space in your dash that is left when you remove the factory radio CD player sat nav and air vent , the kit also means to can remove your iPad very easy when your not in the car , best face off stereo of all time
