[VG2007]

I was looking at installing an aftermarket headunit in my 06 Non JBL Sequoia. The adapter kit I was recommended had 2 plugs, which seem to cover all the basics, but the factory stereo has 3 plugs on the back. I also noticed it has 2 antenna cables. One is smaller than the other. Can anyone tell me what these are for?

Here is the plug in question. It only has 3 wires going to it. Grey/Red a Green/Red and a Brown/White.