Great info!
Years back before I knew about this forum I repaired a horn relay on the 91 Capri my aunt has, electrically it worked but the bottom of the relay case had rotted out. I was able to mend it with epoxy putty and it worked again for some time but did fail a few weeks later. What I ended up doing was to wire in a standard 4 pin relay even though it's a 3 wire part. The OE part grounds the relay coil through the case so just put a little wire with an eyelet on one side of the relay coil and use a star washer to bite into the paint where it bolts down with the eyelet between the relay and the body, then connect the other 3 wires normally. It works fine and the parts are $5 if you need a new relay. I need to update her to a sealed relay which uses weather pack type seals and a skirt to keep water out, at a later date.