The advantage with an AUX-capable radio is that the head unit will display "Aux" as a separate selection from the CD changer. An undocumented (as far as I know) feature is that pushing the CD 1 through CD 6 selection buttons in AUX mode changes the input sensitivity of the auxiliary input.

If you don't have the CD changer, then using the changer input for an auxiliary input would work, except... The radio sends an "are you there" signal to the CD changer and expects a response. If it never gets one, you won't be able to select the CD changer. If the head unit did receive a response but then doesn't receive any further responses, you'll get a "CD Check" error on the head unit.

A number of 3rd-party interface vendors provide adapters that respond to the head unit polling and pretend to be a CD changer and have inputs for an auxiliary device. I know of at least one that lets you use a CD changer at the same time. You select whether you want the CD changer or the aux input via a number of keypresses on the head unit.

I have an 09/2002 build 325xiT (first week of model year 2003 production) and the factory wiring harness had an error where it wouldn't recognize the genuine BMW auxiliary input adapter. Two of the pins were swapped in the wiring harness behind the NAV screen.

If you're connecting directly to the pins on the BM53/BM54 radio, that shouldn't be a problem.

Note that different BMW production facilities phase in part changes at different times, depending on inventory. If a feature is listed as being available at the start of a model year, all vehicles of that model year should support it. However, a slightly earlier build from near the end of the previous model year may also have the feature. If the factory ran low on a particular part, they'd order the newer one rather than more of an older one discontinued by the component manufacturer. In the specific case of the AUX input, I have direct experience with this - a friend had an 08/2002 (2002 model year) BMW and I was able to install the AUX input cable in his car and it worked with no changes.