First time install, some questions

[Wnt2bsleepin]

Hello all. My brother's birthday is in a few days and I wanted to purchase and install a new head unit for him. I found a good one at Crutchfield and I am sure that it is within my price range. The car he has is a 1991 Ford Mustang.

A) The car already has an aftermarket head unit inside it. Do I need to get a new wiring harness with it or will the old one work fine?

B)The old head unit's display is currently flashing, in a broken fashion. I would like to know if there is anyway I can diagnose an electrical problem with the car or if it is just the head unit.

C)The car has 6 speakers, but I am unsure how to wire 6 speakers as I only see 4 possible on that head unit.

I will post a link to the head unit when I am allowed to.

Thank you for your responses.

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[Wnt2bsleepin]

Anybody?