I have a 99 Durango that doesn't have an AUX input. Could I use an AUX cassette adapter instead of replacing the stereo?

Hi yes that is the only way.

Replace the stereo with an AUX. Keep is simple.

I'd put my money on your satisfaction with the cassette adapter. It's simple and works very well. The only complaint I've heard is if the cassette transit running is very noisy. In that truck I doubt it could be that noisy. Go for it.

It always has a hiss. We are so used to good quality now that it is irritating and a good stero is $99. I had an old truck so I mounted it under the dash because I was not going to leave a good radio

yes, if it has a cassette player.

Sound quality will be pretty lousy, but it works.