Help Needed Excursion Rear Audio

[bigbrother44]

I just bought a 2002 Excursion Limited. It has an aftermarket, low-end, JVC radio in it, with a bunch of extra wiring for subs and amps. And as my daughter found, no audio at the rear jacks. I tried the stereo preset 3/5 button combo to turn it on/off and it made no difference. Knowing how finiky dashboards can be about being torn apart too often, I haven't pulled it apart just to look, thought I should know what I'm looking for first. I searched around on here and didn't find my answers, so I have some questions...

Is the rear audio a function of the stereo or the wiring? Or both? I'm assuming that with the 3/5 on/off feature there is something in the stereo that matters.

Is there a particular wiring configuration I should look for behind the stereo to see if it's a wiring problem?

Or is it an "auxiliary output" on the back of the stereo that feeds the rear audio system?

I know other people put in aftermarket stereos, so when you do, what is the trick to keep the rear audio working?

Thanks in advance for any help!