I have a 2002 GLS with Am/FM cassett , I want to put in a CD player, should I go aftermarket or find a VW one ?

I've heard that you can cause problems if you fit aftermarket as the unit somehow upsets the electronics so my advice FWIW is to ask plenty of questions.

Picked up 6 disk changer $200 and bracket $25 from VW dealer. Install was about 30 minutes, flip up carpet in trunk and cut out the area marked for CD changer. Remove small plastic door to reveal connector, mount bracket for cd changer connect changer connector to car connector, put in cd's and press button on radio and presto works perfect. No need to go to dealer to program radio as some may have heard.