Cool, thanks for the info.
IIRC the horn is a ground to the BCM. If you added wheel controls, maybe the horn isn't sitting properly in the wheel/airbag setup. I'd double check that and while it's apart, test the wire for that ground etc. If needed, pull the column connector and check if the horn wire is grounding in the column and then go from there. It's gonna probably be a trace all the way if your horn isn't improperly sitting in the wheel.