Leo,
If I recall correctly, the antenna cable is plugged in at the top right corner on the back side of the radio (large diameter coax style cable).
That's the top right corner as you are looking at the face of the radio installed in the dash.
At least that's where it was on my '81 C10. IIRC, it was tight getting up behind the dash, but doable unless you have factory AC. The factory AC ducting runs below the radio, making it almost impossible to get to without removing the duct first.
A word of caution, there be sharp sheet metal edges and exposed screw threads under there that tend to bite you.
Hope this is helpful.
Happy New Year!
