All of them say to disconnect the battery, just to make sure us dumb technicians don't make a mistake. Still, no reason to even think about removing the airbag in this case, we've already figured out the horn switch is just fine.
I don't know why they fail, they're basically just a computer and I'm sure the solder points wear out like everything else. All it does is take your analog window command from the switch, convert it to a digital signal and send it to the other multiplex unit where it gets converted back into analog and operates whatever, just to save on the wiring.
On most other cars they're called Body Control Module and use a CAN-BUS interface.