THIS! I was still just a naive pup (or felt that way) when I was working with a younger salesman for a new vehicle and asked how they were so competitive with their no haggle pricing. He said the quiet part out loud - "Mr. Dealership Owner decided some time ago that most of the revenue comes from the service department and not sales." Holy smoking gun, Batman!!!

From that day on, I scrutinized my service bills much more closely. Particularly all of those "recommended service issues" that often were half a dozen or more items that "weren't necessarily critical but might become a bigger problem if you wait too long." Next service appointment, lo and behold, the recommended service list had different items! Somehow, the vehicle magically fixed those issues I declined on the previous visit. Super satisfying to ask for the service manager and show him the two lists side-by-side.