[ray_mcavoy]

That 3-terminal horn relay without the bussbar looks like the style that was used on Chevy trucks from 60-62. That same type was also used in 67-72 GM trucks but located under the dash instead of on the radiator support.

The bussbar wasn't needed on 60-62 because they joined the wires together at the BAT terminal on the generator voltage regulator instead. And 67-72 eliminated the need for the bussbar by joining the wires together in a factory splice inside the wiring harness. I don't have any 60-66 GMC specific wiring diagrams to know for sure, but I'm guessing they did something similar.

Anyway, with the relay oriented as shown in your pic:

Wire from horn button connects to the upper terminal closest to the radiator support.
Wire to the horn connects to the upper terminal out away from the radiator support.
Wire from battery + connects to the lower terminal.