Check for 12 volts at the RED wires at the horn relay. One of them comes from the battery, the other has a fusible link. From there it goes to the rest of the car. See if there is power at both ends of the link.

If you have power there, get back to us with a power reading at the junction block behind the battery. That's a weak link there because of corrosion issues.

I have a attached my drawing of the 69 Camaro Power Distribution wiring diagram. The wire colors are usually different, but this should help you visualize how your 68 is wired.