> I was never sure which was the subject (if that's even the right term): 'you' or 'who'.

That's correct. The determining factor is whether "you" is the subject for the verb "is" or "who." I believe it's "who," and that is frightfully unhelpful, because the question remains, what forms of "to be" may follow it?

There is an additional complication. "To be" is a linking verb, which means the equivalent of its "object" actually takes the subjective form: "It is I," rather than "It is me." Normally, when what comes before and after a form of "to be" are in disagreement, the form the verb takes agrees with the subject: "I _am_ your worst fears incarnate." "The Father, Son, and Holy Spirit _are_ one." However, when one or the other is a pronoun, they must agree: "_That_ is my thing." "_Those_ are my things." Using part of your own sentences: "It _is_ you." "It _is_ I."

In the case of "who," however, we find the following correct: "Who _am_ I?" "Who _are_ you?" "Who _is_ that idiot?" "Who _are_ you people?" The word "who" defers the decision on what form the verb takes to the "object" of the verb (I say "object" in quotes, because it's not called an "object" if it's a linking verb, but I forget what the correct term is, so oh well).

The above suggests that the linking verb in "It is you who [is/are] wrong" should defer to "you" for agreement. But "you" is NOT the "object" of [is/are]! The above examples are NOT analogous to yours. In "It is you who [is/are] wrong," there is no noun after the linking verb that sheds more light on what the subject is. And so the sentence becomes analogous to things like: "Who is knocking on my door?" and "Who is involved in this Florida recount controversy?" The answers to these questions could be "I" and "lots of people," respectively, yet neither agree with "is." The second question is particularly interesting because it is KNOWN, from the meaning of the question, that more than one person is involved, yet the verb is still "is," not "are."

And so we can conclude that "It is you who is wrong" and "It is I who is wrong" are both correct.

I just which I knew all the language terms that apply here.