I always thought that is from companies that get their hands on registration data. Or I could be wrong and it is the dealer itself selling it on not the manufacturer.
Pretty sure it's registration data. Anything public is now used for junk. We transferred a piece of property this year and have been getting constant spam for realtors to sell it for us. We bought a used car from an individual and started getting spam for warranties once we registered it and got plates.
Just to be clear: real estate is generally public records viewable by anybody, while vehicle titles/registrations generally aren't (without having been deliberately sold by the state).