that setup sounds sooo ghetto! (joke)
dude, get ya self to a radio shack and ask for a 3.5 to 3.5 millimeter adapter.
its about $6 + tax
and of course plug one end to your radio (wait..do u have a 3.5mm hole on your car radio?) and the other end to your phone. you can even talk on the phone with it plugged up

 

You should stop by a Best Buy, they usually have free/cheap install deals and will at least be able to tell you exactly how much it would cost to get a 3.5 jack (or a new stereo) installed in your car. It might not be as expensive as you think.

 

you can get a decent car stereo with a 3.5 jack for very cheap these days and most places will install it for free or for like 10 bucks. the 3.5 way is just so much better

 

My '96 Audi still has it's vintage cassette so I just use a cassette adapter. How's that for cheap?

 

Pop out the head unit and look on the back..I bet it has the jack or a AUX pinned plug to plug into and convert over to a 3.5.

 

I use a cassette adapter also, for cars that have a cassette deck, works much better then any fm transmitter. My van has the stock radio which has both cd and tape.

 

If you have a CD changer, you can have the AUX out switched from the changer to a 3.5mm jack. You do of course lose your CD changer.

This is what I did on my '04 Yukon.