The overlap report can indicate that a dealer may want to consider signing on with a particular 3PA, but it only shows where the eyeballs are. Those same eyeballs will still be there once the dealer signs on, so how do you tell if it's working? The answer won't be in the overlap reports

Look for a shift in the ratio of third-party only shoppers vs both-website shoppers, notice traffic to the website from the 3PA and look for leads. These are the indicators that the addition of that third-party marketplace is having an effect. Look for a small increase in the percentage of sales influenced.

Were the days to close reduced? Has the new/used ratio changed?