1. Y is part of a cognitive system Z.This seems to me not a correct reconstruction of what Haugeland is up to. So, I take it that it is supposed to be the high bandwidth interface (HBI) between the brain (or what Haugeland calls "the internal guidance system") and the road that makes the road part of the cognitive system. It's not that the brain and something else are both connected to the road by HBIs, so that the brain and this something else are functionally equivalent. Here's a picture of the two different takes.
2. X (an external item) has the same high-bandwidth interaction with other parts
of Z that Y has.
3. So, X is functionally equivalent to Y.
4. So, X is part of Z.
Here's the text from Haugeland (as best as I can get without my hard copy):
What is the Y in this passage?
And, you can ask of the Otto-notebook case, what is Otto's brain and the notebook both coupled to so that they are equivalent?