My 2004 Infiniti G35 doesn't have an Aux input or even MP3 CD playback. It has a 6-disc CD changer and an anachronism called cassette tape deck, which wasn't useful even in 2003 when I bought the car. As a result I had to burn CDs of everything and play them in the aforementioned 6-CD changer, which was neither convenient nor cool.
After several years of suffering at the lack of Nissan Motor Co.'s foresight, I tried a couple of things. The first one was an FM radio transmitter built into one of the Latte MP3 players I tested (Latte Communications Espresso MP3 / Digital Media Player with 3-inch Touchscreen). That was convenient, yet the sound quality was unacceptable.
The second was a cheap no-name cassette tape adapter, which my car rejected by immediately ejecting it. I assumed that I wouldn't be able to use a cassette tape adapter to ply my MP3s. But then, after a year or two, I decided to try a good adapter and ordered this Sony CPA-9C "Walkman Car Connecting Pack for MD Walkman and CD Walkman". Even the name suggests that the product is old, but the result was actually rather good.
The adapter is said to work with the decks that accept the cassettes with the tape facing the side, not away from you. Or, verbatim, "Compatible with car cassette decks that have side tape opening". The adapter has a "self-adjusting spring head and silent mechanism". "Mini-plug inserts into CD or MD player's headphone jack" means any 3.5-mm jack. And the frequency response is rated at 50-20,000 Hz, depending on your tape deck's performance.
Surprisingly, unlike the cheap non-name adapter that my car immediately rejected, this model was accepted and produced only minimal mechanical background noise in operation. The 1.5-m cord is long enough for me to position my Sony NWZ-S639F (NWZS639F) 16 GB Digital Media Player wherever I want.
I disabled Dolby Noise Reduction as the adapter's packaging suggested.
The sound quality is much better than that of the FM transmitter I tried but is somewhat worse than that of the CD playback. In particular, the bass is lacking, the treble is exaggerated and the dynamic range seems a bit suppressed. However, I suspect that at least part of this is due to the mediocre quality of the tape deck. And unlike the FM transmitter that I found unusable due to bad sound quality, this adapter is actually usable, especially when I used my Sony NWZ-S639F (NWZS639F) 16 GB Digital Media Player's equalizer.
Pros: Good build quality, works in my car unlike some generic adapters, good sound
Cons: I wonder if it could sound even better
For less than $12, this is an excellent product. If you don't have an aux input or a USB port, but have a cassette deck, this adapter will provide better sound than FM modulators and won't cost much.