I remember this happening on Grand Am's, not RM's....
Usually happened in the winter time.....when the vehicle got cold, the horn contacts, which were real close, would contract, and this would set the horn off......the contacts were all part of the air bag unit.....
Don't know if this is your problem or a guess.......but there is usually a connector from the airbag that goes into the cancelling cam for the horn......I would have that disconnected(naturally your horn contacts won't work now)......if the horn continues to go off, you know the airbag is NOT THE PROBLEM.....
I've seen crazy things happen....do you have keyless entry? Have heard of other people's remotes setting off the horn.....also, does the vehicle have something like Content Theft Deterrant? May have a shock sensor in the system, that is set too sensitive.....we had a Grand prix once out in our lot....there was a crack in the pavement under the car where it was parked....every time a heavily loaded tractor trailer went by the shop, the horn would go off....
As for rigging a seperate switch for the horn, can't do that in my state......everything has to work as designed......