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.
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