I use a
(no longer available apparently, bought it for something like $19 at the time) for my Garmin GPS unit (with MP3 player) for many years and it works just fine. Others however have really bad experiences with FM transmitters.
The best way, but more expensive is to find an auxiliary input adapter like the ones from PIE and USA Spec.
PIE TOY03-AUX Auxiliary Input
USA Spec: PA20-Toy
While some people go the remove the entire radio route to install these wired aux adapters, it is possible, if your hands are small enough, to remove the glove box (as you would to change the air cabin filter), and reach in behind the radio to plug the Auxiliary Input adapter into the back of the stock OEM radio. These aux input adapters work with the single disk and six disk OEM car radios. From what I remember you hit the CD button on the radio twice to activate the aux input on the back of the OEM radio. Don't know however if the song/track info is transmitted to the OEM radio with these aux input adapters.