Kant and the Prolegomena?

In his letter to Marcus Herz of 1772, Kant posed the problem of how representations that have their origin in the human intellect can refer to an object when they neither are caused to exist by the object nor cause the object to exist. How did Kant answer this question in the Prolegomena? Why did he think that both the rationalists (such as Descartes and Leibniz) and the empiricists (such as Locke and Hume) could not give a satisfactory answer to this question. Thanks!