I had a gizmo that went into the cigarette lighter, and had a USB port. I could load up a thumb drive with tuneage, and the spud would read the files and broadcast via FM to the indash. I wish something like that were around, I doubt that it is. Also, in this car, the cigarette lighter does dead with the key off, so I have to retune every time (I've got some kind of lower-end gizmo now, the cool one died; also the new one doesn't have a very strong signal, so that sucks, and it has a jack to plug in the music source, also sucks).
There are a number of FM Transmitters available on Amazon. One of these might suit you.