https://en.wikipedia.org/wiki/Automotive_navigation_system. Moreover, while often not ideal in dense urban environments(modern receivers often struggle here anyways), by the late 90s differential GPS augmentation was available in cars as well, which was available in dense coastal population areas like NYC. Old auto nav systems were clunky and with overall shitty map data but they weren't "definitely useless" due to SA.
EDIT: I'll concede they were pretty bad, but SA was only one factor. With today's computing power and higher quality maps you could more easily adapt to the SA position error if it were an issue as well.