I've got one of those old tapes with a headphone jack in it that works much better than the two transmitters I tried. They're worthless.

You can also have a car stereo place install a line specifically for iPods that have that connector (don't know what it's called technically) coming out from under your dash, the thing on the left:



A friend of mine has one that allows him to use his car stereo to control the iPod menus. His is expensive, but I think you can get a basic one for less than fifty bucks, plus install.