I think the difference between Locke and Kant is captured in the primary/secondary quality distinction:
Locke's view is just a step beyond naive realism. It is in some sense, our modern everyday view. Although he said we couldn't know the true nature of things, he thought position, motion, volume were primary qualities (qualities that existed in objects independent of the observer). I think this is close to the view most people take today... that there's an object out in space affecting our senses... our senses may not be accurate, but give some kind of analagous copy of the real object. ie: physics deals with position, velocity etc... and these quantities are taken as real and belonging to objects out there in correspondence with the objects of experience.
Berkeley took the step to say that all qualities are secondary. ie: position, velocity etc... are also experience dependent (if you take away all experiential aspects of objects, in what sense can you say it is at any spatio-temporal location).
Kant, imo, kept Berkeley's view that all properties are secondary, but then analyzed these secondary properties in greater detail (analyzed the structure of experience which is all that we can really do). All properties might be secondary, but space/time are preconditions of experience, and have a more fundamental role to play than smell or color. Same with categories of experience.
For Kant, the objects that exist independently of the observer (noumena) are not analogous to objects of experience. To him they were something completely alien we couldn't talk about with any intelligibility. Different from Locke's view.
Did some digging and found this term, 'indirect realism':
According to this, Descartes and Locke were indirect realists.