[SOLVED] How to use AUX (stock sound system) in Chevy '99 Lumina?

My guess is if you don't see a port on the face of the stereo, there isn't one at all. Like most electronics manufacturers, it is cheaper to leave the buttons on even for optional equipment than to take them off. So you probably have a button that doesn't do anything unless you have the upgraded model of the stereo that includes the AUX input hardware. I've only seen aftermarket head units that have AUX inputs on the back. Stock stereos weren't intended to be removed ever unless they're being replaced, so putting an input on the back for future use wouldn't make much sense. I don't have any experience specifically with your stereo, but I do with lots of GM / Delco stereos.

I agree that I've not seen GM systems with inputs intended to be made behind the bezel, but I disagree about extra buttons. I can't think of a GM factory system, or any other that had extra buttons. Generally, the CD/AUX in a GM interfaces to either a single CD built right into the dash, or a changer - both intended to be factory options. There is some information fragmented around the web on how to interface other devices to this, but I didn't see anything consolidated. The OP is going to need to search the web and put together the info he needs.

On the Gm units the Aux. button had a couple uses. If you had the optional changer it selected it, or if you had the remote mounted tape deck (like the 02 Blazer we have) it selects that. You have to buy an aftermarket box that will connect to the data buss connection on the back AND have a dealer program the stereo so the button will activate the new box. Then you will have an aux. line level input.

I have the exact same car (99' chevy lumina and this is a delco stereo. I want to get apply a aux cord on here but i have no luck searching up a way.

On the AM/FM Tape units that button is used to put the tape deck into aux mode. However it doesn't have any connections to tap into for a cable, it is intended to be used with one of those cassette adapters with a cable on it or the modern ones that have bluetooth connection. To use it you turn the key to run or accessory, Turn the radio off. Now press and hold the aux button for 5 seconds, The tape light will flash for 2 seconds. Now insert the adapter into the radio, it will power up the radio and you can use the radios controls to listen. The Aux mode will reset to tape when you push eject.

On the AM/FM CD units it is also not connected to any external wiring and just selects the CD if you are in radio mode. Or on some models it allowed you to select a remote CD changer or a tape deck. There used to be a kit that you could install inside the radio that allowed you to put an audio signal into the system but you needed to put a blank CD in as it tapped into the CD signal feed at the internal amplifier to work. Not sure if they are still around or not.

Thanks dude, I have 2001 lumina and an old Sony cassette/with 1/8 stereo plug now I can use my phone for music with a 5 sec push on a button that appeared to be useless wow thanks again!