[IlovemyFX]

Definitely a coaxial connection. maybe the original owner had a CB radio antenna mounted to the rear somewhere. I had a CB in my early 90's Ford Ranger 4x4.

 

[IlovemyFX]

Well, it does look factory but I've never seen that kind of connection used before. Does the car have satellite radio?

I would suggest pulling up the audio and/or electrical service manuals. You can get them over at Infinitihelp but will have to pay a subscription fee to get the entire manual for your Q45. I think Nicoclub still has service manuals available at no cost as well.