I had this issue when I wanted to connect my ipod to my car. I tried a couple FM transmitters and one worked OK but not great. My wife's car had a casette tape and I used the plug in adaptor for that and it was OK. But I really solved my whole problem for not a lot of money when I got a JVC KD AR 560 from Crutchfield and installed it myself. If I can do it probably anyone can. It was $200 and better than the original stereo in my 2005 Ford Focus which had a decent unit in it.
This unit is one of only two I found with the 1/4 " imput plug ON THE FRONT where they all should be. There are various sound adjustment profiles which will work with different imput signals like from a cassette, ipod, MP3, or I can even hook up my laptop to play into it. I suppose my old Nomad MP3 player would work but after getting the ipod I don't bother with it. CD quality sound and it will work with Sony mini disk players or anything that comes next on the market. Now it is not as snazzy as those units which control the ipod from the head unit but so what I can pick up the ipod (or the cassette player I use and look at it right in my hand.
A fair number of car stereos have various plugs in the rear of the unit but that requires an adaptor cable and there were other problems I encountered with that approach.