2008 Freightliner Columbia Day Cab replace air horn

I have a 2008 Freightliner Columbia Day Cab and I need to replace the air horn located on top of the cab. I'm trying to figure out what all I need to take apart to access the hardware holding it on. I don't see many screws or anything holding the plastic mouldings on and I pulled down on the carpeted headliner only to find more plastic behind it. The truck was parked in the way of an irrigation pivot and the end of it hit the top of the cab and broke the windshield, dented the top of the cab and broke the air horn. They horn still works but does not sound the same with the trumpet not attached. Any info is appreciated, thanks.

 

I did stand the square piece of the air horn up like it is supposed to be. The mounting hardware is a flat piece of metal connected to both bolts and there is no head to attach a wrench onto the fastener (pictured below). It can only be removed by taking off the nuts located above the headliner and under the roof. Thanks for your reply.