Wow, thanks for that. That is a great visual of the crazy HVAC system of mystery.
I have been all over under the dash and cannot for the life of me find anything that looks to be the receiving end for this plug. The area you described sounds like this...
That's already occupied as well....I gave up looking and installed some RSA Door panels and rear seat delete carpets today. Haha.
My car will be in the shop soon for some suspension upgrades. I'll bug my mechanic about it.
Thanks for the suggestions.