I have an 87 coupe. I had to replace My relay and horn fuse recently.
The fuses were not anywhere the manuals said they would be originally (like on the drivers side or passenger side), but where smack bam in the middle of the center dash cluster behind the warning display panel.
I discovered my relay and the horn fuse is behind the centre stack in the dash, behind the engine warning display screen/panel. I took the center fascia off, and then that allowed me to pull out the dash display flick switch cluster and the warning display screen/panel.
And there it was! Also, a small fuse panel containing three regular fuses is located behind there too. One of those three fuses was a 15amp fuse for the horn too, and it was blown. This was the cause of my horn not working, but i replaced the relay while i was there because it was a pain to work on.
The only way i discovered all this was because my horn relay was still clicking when the horn was depressed, but there was no sound output due to the fuse being toasted. So i scoured the dash with my ear for where the clicking sound was coming from and discovered all this.
Hope that helps