I want to install an AUX cable into my 2008 Prius base stereo

Hi there,
I've been reading through existing posts about the AUX cable, but I still have questions, and some of the most relevant posts are 3-4 years old.

Just last week I bought a 2008 Prius base model, and the stereo has no "Aux" button, nor does it have a Aux cable port. I've seen some posts and picutres showing how to add an Aux cable to pins #16-19 on the back of the stereo pin. I have not yet taken anything apart other than the glove box....I would like to know more before dismantling anything.

Q #1) Does the white plastic plug on the back of the base Prius stereo have empty pins for #16-19, or is the problem that the plug for the base model doesn't even have those pins? The user who took pictures reported buying a different stereo from salvage but then returned it when the CD player wouldn't eject, and re-installed the original stereo...which leads me to believe that I can make my existing stereo work.

Q #2) If I connect the Ground pin to the "Aux sense" pin (as one post suggested) does that mean it is always in AUX mode?

Q #3) And if there was a switch installed in that cable, does the On/Off capacity of the switch enable the stereo to exit out of Aux mode back into FM radio mode?

Thanks,
Diana