Depending on your dash radio there are several options for feeding music from the I-Pad to the Radio, Some work better, one can cause issues, if it does choose another.
First: IF the radio has an "AUX IN" this is usually a 1/8" mini stero phone plug... (Like the h eadphone/external speaker jack on the I-Pad) most electronics stores, Best Buy, Frys Radio Shack et-al, will have the double male patch cord. Very common, this is the best quality feed. HOWEVER..... In some cases if you plug the I-pad into a car-power adapter it will cause issues.. Solution 1. Run the I-pad on internal batteries or on 120 vac via an inverter.
Option 2: Vintage radios with built in cassette player.. I have the adapter in stock and can send it to you Just send me a check for whatever the postage stamp is after you get it. (I'm not kidding that's all it's worth) if you are near me it's yours, I'm not selling it, Just handing it over (Assuming I can find it). Some stores still cary them.
Finally, there is the I-Rock or I-beam or simular adapters.. These are small Stereo FM radio transmitters, they come with the plug to match the I-pad/device, and transmit on either a variable frequency or one of several fixed frequencies.. As I recall the I-Rock runs on a pair of AA or AAA batteries, and the I-Beam has the option of a car cord.
again with any device that uses your vehicle's power if the I-Pad is also using vehicle power you can have a problem... Run one or the other on batteries to cure it.
The problem is noise by the way.
Now, finally.. Most of the dash radios sold today, either have the AUX-IN jack as described above or have some kind of rear panel jack, the details of this jack will very from make to make,, Kenwood for example has a round multi pin plug, Pioneer it looks more like a phone or lan jack.. They sell assorted things that plug into said jack.
A multi-disc CD changer
A Satellite Radio Receiver
An "External Input" (AUX IN) device.
On the Pioneer, the first two have daisy chain jacks so you can plug in all 3.
Then (you will need the fine manual for this) you go into the menus and turn on the device, select it and feed away.
Last Sunday. I had a recording of a radio broadcast on Mini-Disc,, I popped it in my Sharp MT-15 and plugged that into the aux-in device on my Pioneer.. We enjoied some very good classical music (FIrebird suite as performed by the Detroit Symphony) as we went to church.. It is how I do it.
If I really wanted to get fancy.. I got the hardware.. but alas I won't even describe that (IE: Mix the audio out from the computer running Streets and Trips with the Mini-Disc player or other device.. I can do that. but that's because I used to be my daughter's recordign engineer so I have the very nice stuff to do it with)
Home is where I park it!