Batboard

I am trying to add a second Mic to the rear of a Chevy Suburban using the accessory plug on the rear and I am having no luck, can anyone assist with wiring information for this project?

I used pins 2,3,7 on the 16 pin accessory on the rear of the radio and put them in the jack that was made for the rear of the unit. Didn't work. The CPS says that only pine 3 is needed for external ptt. Need help all confused!

Follow up, currently have the radio to key from the rear of the vehicle but it will not pass an audio! Still looking for any input, suggestions or help would be appreciated.