Hi, sorry to hear about your troubles but I have seen it before. It is the main reason I only use the factory steel line on the pressure side of the pump to carb. and not rubber. Even with a small stream squirting out from a bad connection or cracked hose the engine will still have enough pressure to stay running and keep pumping that stream of gas all over your engine until it finally catches fire. As far as the third wire that is the main + wire from the stud on the back of the alternator that goes to the battery and charges it. from the diagram you have it is hard to tell what that other thing is near the relay unless its just shown separate to show the wiring for the key buzzer that is in the originals. I don't believe the aftermarket relays have it. I did not take the time to go through a whole diagram but here are some from Harold B's site, and a detailed picture of the horn relay wiring with warning buzzer. The only other thing I can think of is mounted on the firewall above the master cyl. and is used if you have a pwr. conv. top and or pwr. windows but the full diagrams on Harold's page will tell the tale. https://www.firebird400ho.com/mdocuments-library/ I just noticed on the bottom of your post it says 67 FB if 67 they did not have the key warning buzzer so make sure the diagram you have is 67 only and not a hybrid.