Hey, people how's it going? Hope you're doing fine! Do you know if it's possible to just plug in a CD stereo from '93 Mustang into a '90 Mustang? Right now I've got just the Cassette/AM/FM deck, but I can get a '93 for virtually nothing!
The only change from 1990 through 1993 in stereo equipment was the optional cd player that you're speaking of. This would not necessitate any change in wiring as the speaker quantity and locations did not change. So I would say that you should be fine plugging the cd player right in. If by some chance you can't, the wiring is very simple and all you need is some wire crimpers/strippers, some crimps, electrical tape (or heatshrink if your a perfectionist), and a wiring diagram(easily found for both 90 and 93 on the internet).