It was you who ____ wrong.
we used who because it refer to you
It was you who was wrong, wasn't it?
The "was" goes with the "it".
oh my!. I don't understand why so many people said "was". You is always followed by "were"!! Am I wrong??
"who" has the he / she / it function. that is why "was"
Who was, if talking to one person, you were if more than one.