I'd suggest looking into a new head unit first. It may be a little more expensive than other options, but you'll get better sound quality than a factory stereo, even when listening to CD's and regular radio. They also have more flexibility, like with aux inputs, USB connections, and Bluetooth. So the extra cost is worth it.
There are also FM modulators that hook inline with your antenna wire, and those give good sound quality. These aren't to be confused with the crappy FM transmitters that plug into your device and broadcast a weak FM signal. What I'm talking about hardwires in, so there's no signal loss or interference. They are kinda pricy, though, from what I've seen.
They come in two flavors. One would have an aux cable that connects the modulator to your device, much like a cassette adaptor. The other is a Bluetooth connection and can be 100% behind the dash with nothing exposed, except maybe a power switch for it. I tried a Bluetooth one before on a factory ratio with mixed results. I could pair my phone and listen fine when testing. I had planned to use it with my SiriusXM tuner, but that wouldn't pair right. I never could figure it out and eventually returned the modulator.