From what I understand some modification has to be done behind the deck with some aftermarket headunits due to depth issues. Door chimes, OnStar and RAP (retained accessory power?) you know how your radio stays on until you open the door? I think you lose that too without the special harness. I just ordered the Metra GMOS-09 harness so that I keep my chimes and RAP. You would also need an antenna adapter for a VW not a GM, I believe the part number is VM10. I am definitely not the expert when it comes to this since I've never put in my own radio, and all the parts for mine are still being shipped to me.