Thanks Brian. Great info ..Sure Mark - You can bench test the relay function with a 12volt power supply and then grounding the horn lead, the relay will audibly click when it closes the circuit. And with a multi-meter you can test continuity through the power routing portion of the relay.
The points on the relay if found to be in rough shape can be cleaned up with a few passes of nail-file emery board.
If in less than show condition the metal embossed cap can be carefully removed, soaked in evaporust then polished and then reassembled.