80-82 Radio Wiring Question

[Dennis G.]

I am replacing an aftermarket radio in my wife's 80 Corvette with one from a 1981. I don't know what radio the 80 came with originally but it has speakers in the back. The 81 radio has a two-wire connection with a small plug that has an orange and brown wire. I know it's a memory feed and the orange wire is a power wire, but don't know where the dark brown wire goes. My Shop Manual shows the wiring diagrams of three different radios but not the one with a cassette.