2000 ES300 and Garmen GPS C550

I have a Garmen GPS C550 with a built in MP3 player and when I use the MP3
player the sound is not good.

Is there a way to hook into my car stereo when playing MP3's? My GPS has
bluetooth too so could I use one of those bluetooth cassettes and use the
audio out on the GPS? If so where can I purchase one of these that would
work with my car?

Any help on this greatly appreciated.

Thanks
Mary