Very confused on differences between headunits & inputs (CD changer, AUX in, etc?)

Hi

I have a 2002 325 convertible, HK system, business head unit, no CD changer, no nav. I've been researching aux-in options that I have, and I see that everything says that a simple aux-in cable is only applicable for head units manufactured 9/2002 or later. What's confusing is that my head unit has two inputs in the back, once is taken by the amplifier plug, the other is open and unlabeled. My guess is that it's for a CD changer option.

I've seen a number of posts on using an aux-in 10-pin cable to 3.5mm cable and plugging it into the CD changer/AUX input in the back, but I'm wondering if because my head unit was manufactured prior to 9/2002 if it'll actually work? The input on the back of my headunit isn't labeled, so I don't know what it's actually for. If it's for a CD changer, does it detect the signal from a phone and just "work"? Or because it's not AUX-in, do I need a separate module?

Any insight would be great...I can't find a straight answer in all my searching.

Very confused on differences between headunits & inputs (CD changer, AUX in, etc?)