Iphone won't work through plug-in cassette on radio?

I just recently got an iphone. Its not a newer model. I think its a iphone 2?
Anyways, I have a plug-in cassette that has the wire with the jack thingeh you put into the headphone outlet… It's worked with all my other phones and mp3 players. It won't work with my iphone. The music does not come out of my jeeps speakers and only comes out of the phones speakers. Anyone know why this is? If so could you tell me how to get it to work? If not… What do I need to buy so I can play music through my jeep with my phone.

Well it won't work like that because of the new connecters they see you plug them into the cigarette lighter plug in and put your phone in it through charging 30 pin adapter so you can buy them at most electronic stores and play it through your radio in your jeep