It isn't hard to swap a relay. Hijacking is when you post off topic" taking over" or draw badly away from the OPs ability to solve their issue. Reviving old topics for continuing it isn't a highjack if on topic.
Starting a new topic is no sin sure.... fyi, I thought from the wording you had already posted about this. Not enough info here to make it easy on anyone trying to help.
.....Or I might know, or others too. and no a pc's dc relay won't be able.to tell ac from dc. Nor any trick to. The closest thing was old telephone relays(where lionel got the shunt relay originally) Knowing how to address most issues and recalling exactly what's there aren't the same thing all the time 
Solder the relay post.... loosing a connection at an old rivet can sometimes be fixed by solder, yes. This applies quite often actually, so backtrack along the battery traces/rivets/connections too.
Voltage crosses bad connects easily until the amp draw rises, then the volts may drop fast where there is some resistance to amps. Testing volts while the horn sounds (loaded); test at the horn terminals too.
You aren't looping any wires near the relay coil are you?
edit: auto.composer added an extra "voltage" after amps...now removed
