Quattro rear differential control relay Up for sale is a Quattro rear differential control relay. If your rear lock-up is no longer working, or failing to deactivate, this is one of the potential culprits! These are great to keep as a spare too even if you don't need one right away, they are over $360 at dealers, and is a dealer only part. This comes with a 30 day functionality warranty from ePartsland!Thanks for looking!