I'll try my best to help you out here. 69 Firebird horn relays are unique and hard to find. I'm not sure what an 875 is. I used a 69 Cadillac relay on my black vert because I got a good deal on an NOS and it was a close match.
Wiring: You have a larger black wire that is black with a single red stripe that gives power to the relay. It goes in the center tab on both the Caddy and Bird relays.
You have the dark green wire and the black wire that go for the horns and they connect towards the bottom or far left depending on if it's mounted.
Then you have the black with pink stirpe that activates the key buzzer. It's on the top or far right. Hope this helps!