What does Kant mean by "existence is not a predicate"? My understanding of what Kant meant is rooted in my interpretation of Heidegger's response to the same question as set forth in Basic Problems of Phenomenology.
First, by existence Kant means "actual" as in currently existing in time. In that sense, that an object "exists" simply means that it has a location in space. And where an object is located would generally not be considered a quality of the object.
Second, by "real" Kant essentially means having primary (and presumably) secondary qualities. And if one were to make a list of Aristotelian or Lockean qualities as applied to the realness of objects (such as color, shape, weight) "existence" or "location" would not be on the list. For example, whatever qualities are essential for a book to be book, where the book is located will not be one of them.
So in that sense, the color, shape and weight of my auto would all be considered qualities while the fact that it currently exists in my driveway would not be considered a quality. The qualities are real predicates of the car while its location is not.