In my experience, FM adapters are terrible. I have had 3 or 4 of them over the years, of different brands, and they all universally sucked. In a rural environment, performance is mediocre. In a city environment, performance is zero.
Cassette tape adapters are MUCH better than an FM adapter.
For $20 or less you could buy an FM transmitter and a cassette adapter and see which one you like better. In my experience cassette adapters provide better sound quality.
Edit: Yeah, what neuroman said.
If you buy a cassette adapter, be aware that they plug into the headphone jack. Most of the time the sound is acceptable through the jack, but you do have to adjust the volume on both the iPod and stereo to strike a good balance. I found one cassette adapter that has a bracket that the iPod’s dock connector sets in and plays the music through that. I don’t know how well it works. It’s a bit pricey at $39.99 and looks a bit fragile. It’s also supposed to work with iPhones up to the 4S model and will charge the iPod as well.
I had decent luck with my old Griffin brand FM transmitter for my Sansa. Within the actual city of Chicago it wouldn’t work but out in the suburbs I could usually find some open band.
Here’s a site that tells you what bands are theoretically open in your ZIP code.
Of course, as you drive around you’ll find yourself looking for new bands because yours just filled with static or you’ll get interference from other cars “broadcasting” on the same frequency. For a long term solution, you’ll want to pick one of the ways to physically connect your player to your stereo.
I’ve never seen a stock head unit with RCA inputs. Aftermarket units, sure, but that’s not what the OP has.