If you have the stock stereo, then you should have the AM/FM/CD Cassette Stereo.
This is definitely old school, but you can't beat the simplicity and the cost. Amazon.com sells it for $11.76. And it's a genuine Sony product, not a no-name brand.
http://www.amazon.com/Sony-MiniDisc-.../dp/B00005T39Y
The sound quality is obviously limited by the cassette deck, but I've played cassettes on my stock stereo and it's really good...
Keep in mind, if you've played a lot of tapes in your tape deck over the years, you should clean the head first.
Also, unlike FM modulator type adaptors, you will never have to worry about interference.