Why is this happening and how do you fix it?
Check for a bad wire from fuse to stereo. Check for a bad ground wire or ground connection. You can remove the stereo and connect it directly to the battery (using a fuse on the power side). Stereo works great, you have a wiring problem. The truck is old. You can get some new wires and connect the stereo directly to the battery using an inline fuse. Put a toggle switch to kill power when you are not using the stereo.