I had this problem and also had a spare stalk sourced about 15 years ago.
Fitted the spare stalk and found that I now had 2 horn switches, one switched the live and the other switched the earth, 1 extre relay sorted and now both work.
So I took the old switch to bits, if you do this split it in a bag as bits go everywhere. I found that the plastic had aged (doner was 1983) and brittle. There were 3 copper bits that had been formed with a pip to interlock with the nylon carriers to allow assembly, the nylon carriers are now hardend and worn, I have only managed to get 2 positioned for reassembly at a time, getting the third locked often results in both the other 2 pinging off.
I have not given up yet, it has passed several boring hours of covid isolation.
I will attempt pictures latter.