Horn wiring

Hey fellas, I'm running into a similar problem. I have a 69 valinat that's been apart for a long time. Horns are last thing to finish to get an inspection done so I can drive it this August. Cant find a relay in the drivers side kick panel. there is a plug that would accept 2 prongs, but the relay I found is 3 prong. Does the 3rd prong just get grounded?

Sorry to hijack the link....