1999 Radio / Onboard Diagnostics Problem

I have a '99 that had a factory radio with a cassete player. I had Best Buy install a CD player - Now The Onboard Diagnostics doesn't work VW says it is not their problem and they know nothing. To get thru inspection in NJ, I had to remove the radio. Does anyone know what the radio has to do with the computer or what the fix might be????

Thanks in advance.

Do a Google search on "VW radio K line".