Dumb question, but I have searched high and low and cannot find an answer:

Do the Bosch or VW AFR sensors simply screw into the existing stock rear AFR sensor bung before the cat? Or do you have to weld in a new bung?

If your talking about a wide band sensor (not stock replacment) they yes you would need to get bung welded in the DP after the turbo, I think it's 6-12"s,