73 duster horn relay ?

i'm trying to get my 69 valiant on the road after being apart for 9 years. horn is the last thing I need to get the inspection complete...
stock stuff. where is the relay, drivers kick panel? Googled it and its 3 prong, looked in that area and I found a 2 prong plug with nothing in it. Does the 3rd prong just go to a ground, or should I be looking for a 3 prong plug? Looked in the service manual and cant find the info I need