Late 90's GM Stock radio Guru out there?

Hello! I have a unique project I've been stumped on and was wondering if anyone could point me in the right direction regarding installing a STOCK Pontiac radio into a car I'm fixing up.
The radio is from 1998 (UP3 option AM/FM CD and 5band EQ). Part# 16228082

Why go stock?? quick answer, looks better in the car and matches the interior and style.
Although I mentioned I liked the Stock look, I really wish it had at least an 'Aux' jack, which obviously it doesn't.

I know of hacks on the internet of tying into the L/R channels of the CD player internally in the radio, but you also need to have a CD in the player to switch it over to that channel to use the AUX input. Kinda lame to me....

I noticed on the back of the radio there is separate 9 pin connector on the back which is used for external devices (like a 5 disc CD changer/Cassette Tape deck/SiriusXM etc..). GM calls it the "Entertainment and Comfort Bus" or E&C for short.

I managed to find an external GM add-on tape deck at my local wrecker with the 9pin harness cable. If I hook it up to my test radio (another GM stock unit but a different model) then put in a tape while the radio is on, it switches the output automatically to the tape source. Eject the tape, the radio channel comes back on.

I want to copy this behavior but with a switchable AUX input. Meaning, instead of having the tape deck hooked in, I want to use this 9Pin connection to 'trick' the head unit into thinking a tape is inserted, switch to the Tape Channel for output when an Aux cord is inserted into the jack.

I know... "Just buy a Head Unit that already does this". (see above remark about why I want to keep the stock look) :)

Has anyone figured this out? I've seen a few posts here and there on these interwebs about the same thing, but no solutions...

A while ago (10 years??) a company made devices that you could insert into the 9 pin E&C bus and it would provide Aux and BT features for the radio. Believe it was called the AUX-9 or something like that. They're not made anymore, and having no luck finding a used one.

Thanks for reading, and here's to better years ahead!

Cheers!,

PK

The head unit does a "handshake" with the connected device to know it's present and access that source. The AUX9 emulated that handshake and used the audio input for another external device.
Use the tape deck you have buried in the dash as the AUX9 and interrupt the audio pins to be your aux input.