Hi!
The actual radio unit itself is not located with the "head" unit or the tape player. It is located underneath the dash--on my dad's '94 it is just about right behind the ash tray door. Your truck is probably similar. You should be looking for a long, thin and kind of silver in color metal box. This is the radio unit itself.
To find your radio, I would suggest opening the driver's side door, getting down on your knees, and looking up above the pedals. You should see it up there.
It will have a bunch of connectors that plug into it. One of them is the antenna. To get to these connectors, you may need to pull the radio down. If I remember correctly, the radio is held by a few bolts. When you get it free, it will come down somewhat...just enough to unplug the cables.
I don't ever remember the reception being very good with the radio in my dad's truck. It was marginal on FM and useless on AM. We had the dealership look at it numerous times and they even put in a new radio after agreeing with us that it was not working right. The new radio didn't do any better, so I dropped it down to have a look. The antenna worked fine with a radio I brought outside, so I can only assume the fault is in the radio. These radios don't seem to be of the best design quality and having as much distance as they do between tuner, amplifier and tape deck cannot be helping things.
I would strongly suggest disconnecting the battery while you work on this.
William The Guesser