I've seen a few threads discussing trimming the plastic around the bag to enable the horn to work. I couldn't find any pictures or detail though. Over time the cover shrinks, and the plastic under becomes stiff and brittle. This makes the cover less flexable, and contact can't be made with the horn switches underneath.

I installed a set of switches as mine were broken. I re assembled everything and the horn wouldn't sound due to the cover being so stiff. As I played with it, I started to break some of the plastic off with my hands, it was extremely brittle.

I noticed the rubber bumpers on the backside of the cover had been pulled behind the plastic, due to the shrinkage of the cover.

I decided to trim it using a razor knife. I went slowly only doing a bit at a time, until I had enough flex to engage the horn switches. I installed it several times until I removed enough material to easily engage the buttons. I WISH I had taken before pictures.

Anyway, this is how much I had to take out, in order to get them to work. I didn't see an alternative, while retaining the stock wheel.

 

Yeah, I did the same thing but even a little more. I had the same problem with the horn, where getting the horn to work was nearly impossible without gigantic force which ultimately led to broken horn switches
On mine, the upper edges were curling up a lot do to the sun warped yellow backing, but after trimming out lots of that and occasional pushing down on it while it was warm from the sun got it to sit flat again. May not be all original but a win-win as far as appearance and function go
I used my trusty diagonal cutters though, much easier

 

Had to do this on my Camaro as well, but still couldn't get the little rubber "square" parts of the airbag to reach the horn contacts. I ended up attaching some 10mm nuts to the airbag squares with some balled up 3M Strip Caulk so that they'd meet the horn contacts. Been about 2-3 years since I did that, horn still works great.