like what parts guy said, but an over-weight/over-speed warning device may not be related to the make and model of the truck, it is often an added 'aftermarket' item.
you need to get your supervisor and the service department involved to be sure it is not a bad (shorting out) horn button wire or the switch itself. a horn that blows when it should not is ever bit as much a hazard as a horn that does not blow when it should.