Quick additional tests this morning. Confirmed that with the horns activated at the steering wheel I get the full 12 volts power at the connectors for each horn.

Therefore at this point I have ordered new horns.

Will install the new horns and hopefully the steering wheel horn switch wont activate or get stuck on its own again. If it does, I know which fuse to pull (wife does to as she is the one that drives it the most).

Thanks for your help guys - I will hopefully confirm that the new horns resolved the issue next weekend.