Hello, I am trying to hook up my 400 hood tach on my 1969 Firebird 400. When I bought the car the tach was already on the hood but was not wired. After some research I found that the tach needs to have ground, 12volt power, and a 12 volt connection to the distributor. I am having trouble identifying which cable is which, ground, power and distributor, can you advise?

The three thicker cables you see are two black, and one brown, any idea? thanks in advance!