what year mustang has the back window fins

Dear missanna2-- The Ford Mustang 2+2 fastback made its debut in 1965 and featured distinctive ventilation louvers, which I think is what you mean when you say back window fins. Am I right? Good luck from Bovan.

The 71 coupes have something like a fin that extends from about the back window to the rear of the trunk...this could possibly be what you are talking about

are u talking about the window louvers.. like the black things on the back window or like the svo wing john is talking about?

Yes the louvrrs

Louvers are aftermarket.