A six-CD changer unit in a Volkswagen can experience a myriad of problems, as with any other electronic device. To repair the problem, you must first troubleshoot the unit.
Take the player apart by removing the screws with the appropriate screwdriver. Remember to save the screws and place them in a safe place.
Spray the inside of the player with air from a can of compressed air. Do this until loose debris and dust is fully removed, as this can cause the CD to stall or lock up while playing.
Wipe the unit's laser lens with a isopropyl alcohol soaked cotton swab. Wipe the lens to clean it of any dirt that may be blocking the lens from properly reading the CD discs in the player. You can also use a CD lens cleaning disc. Insert the disc in the player and press the play button. The disc will clean the lens in several cycles and stop on its own.
Make sure that the problem is not with the unit's fuse. If you see that the filament in the fuse is severed, along with grey film on the fuse glass, it needs to be replaced as it is permanently broken. You can easily find a fuse compatible to your six CD changer unit in a hardware store.
Inspect the wires in the player and make sure that they are connected and secured. In addition, look for any surface damage to the wires. Inspect the gears and gear belts. If the gears are dried out, lubricate with some WD40 oil. If a gear belt is popped out of place, put it back into place on a gear.
Insert the screws back into the unit. Play several CDs in the player to check if it actually works and if the changing process runs smoothly between discs. If the changer still gives you problems, bring it to a professional repair shop, so that they can fully fix the problem.