radio wiring

I have a 91 Bronco full size. I got the truck over 2 yrs ago. The previous owner had some old crappy radio in it. The radio that he had in it was just spliced into it, with no aftermarket harness. My question is, I had bought a nice radio, and I am great with wiring etc, done these before lots of times. I have the dimmer wire off the radio, does anyone know what color wire that attaches to from the truck side. If that wire is spliced into, will I be able to dim the radio with my headlight dimmer knob? I just wasn't sure if those trucks had a dimming wire near the radio or would I have to run one straight from the knob.