Is it possible to find an aftermarket stereo to work with the disc changer? I have a lot of CD's and my 1999 ES300 has an aftermarket POS stereo that doesn't work with it. Would I have to take it to a professional or get some other some components like another unit/programing stuff.

 

Instead of going down that road rip all your CDs to wave files using this program
https://www.exactaudiocopy.de/

Then you can put them straight onto a USB stick, or convert to FLAC or mp3 put them on your phone etc. far more flexible.