1. There was no horn relay used in 1945 for single horn. Relay was used on dual horns. (see note)
2. Wire the relay you have according to the diagram as posted by Dragsix and reposted by Phil.
3. Depending on how you check voltage, you will see some at S when horn button is not pressed. Call it "backfeed". It's OK. Your meter/test light is providing ground.
4. The relay itself is not grounded. Does not need to be grounded.
NOTE: The dual horns, with relay, were grounded at their mounting bracket and had only one wire, which went to the relay.
The single horn, without relay, had two wires, one to hot and one to horn button. (Green51Chevy diagram)
If you have only one horn and want to use the relay. It depends on the horn you have.
Horn that is grounded by it's mounting and has only one wire, wire per diagram.
Horn with two wires, find which is the ground wire and attach to nearby chassis metal. Wire other wire per diagram.
