By referring to the subject on "Who and Whom" on the following link:-
http://www.grammar-monster.com/lessons/who_whom.htm
Here is the two examples:-
I wonder who is in charge.
("Who" is the subject of the verb "to be"; i.e. "who is".)
Who are you going to believe, me or your own eyes? (Groucho Marx quote)
(This quote by Groucho Marx is incorrect. It should start "Whom are you going to believe". However, as "whom" sounds a bit contrived, we'll let Groucho off this time.)
My question come below.
I don't understand why the quote by Groucho Marx was incorrect? Its just similiar to the first example, "Who" is the subject of the verb "to be"; i.e. "who are".) Please explain or did I miss out something?
It's a good question, but you are missing something.
Who is not the subject of are. You is the subject of are. You are going to believe whom? is another way of wording the question. It's question word order that it throwing a spanner in the works for you, but the bottom line is that who is not the subject of are in the Groucho Marx example.