Got everything ready for the new switch/relay combo. All the connectors are in place so all I need to do it screw it into the air line tee and connect 4 wires. Tracking shows that it arrived at the house today.
I did pressurize the system to check for leaks again yesterday. Connected the air horn and, for the first time in 2 years, it blew loud and clear. Had to replace the air solenoid and both electric horns since they were damaged when the horn switch shorted out and kept them on until the battery died while I was away. Horn switch problem turned out to be a cracked turn signal cancel switch in the steering column. Had to remove the steering wheel and the locking ring to get to it but it was a common part and otherwise an easily replaceable part. Autozone's tool rental program really helped out. Was able to rent both the steering wheel puller and the locking plate compressor for a net $0.00. You have to pay for the tools in advance as if they were new, but they credit you back when you return it. I've rented a bunch of tools over the years.
Finally getting ahead on the repairs. Seemed like it was something new every time I checked. The "TO-DO" list is getting smaller now. Items remaining are getting less critical but I'll be diligent since I'm on a roll now.