E20: Focus Error
Cause(*): The CD player cannot focus on the disc or there is an internal focus problem.
Correction(*): Try cleaning the disc or using a new disc. Make sure the disc is loaded correctly. The road could be too rough or there is too much humidity. If the problem is not corrected, service the unit at an ACDelco(R) Service Center.
Since you said that it works on new, store bought cd'* i don't think its the dirty disk problem, it sounds more like an internal focus means that the laser is out of alignment (you can realign it but you have to take the radio apart), or that the laser is dirty. In the later case you could get a cd laser cleaner for about $12. it'* a disk that has a bunch of small brushes on it, you just put it in the cd player and let it run it'* course. Some cd cleaners have solution, some don't. I don't think any one is better than the other.
Try the cleaner first, it'* cheaper and quicker.
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock