Ignition accesory function

[67ChevNut]

While installing the radio I noticed that my accessory ignition switch function is not working. From what I understand, with these trucks, you should be able to listen to your radio, turn it on and listen to it when you turn your ignition switch to the accessory position. So why isn't working? Where does this wire runs? How do I fix it?

I would like to have an accessory function on my ignition switch.

Thanks for your input.