I just replaced my horn relay. I have a 1985 GSL, but I think this will be similar for most 1st gens. This post is intended to help anyone else that has to do this.
First, check the screws that hold the horn button on the steering wheel in place. Roll the rubbery cover off of the horn button on the steering wheel and make sure all three screws are tight. After 30 years mine got loose and caused the horn button to continuously make connection. I unplugged the horns, but could not immediately deal with the issue. The horn relay shares a fuse with the brake lights, so I could not just pull the fuse. The horn relay is not meant to be on for hours, so it burned out. Rolling the rubber cover off and tightening the screws did not take long. Should have gone ahead and done it when it happened.

Where is the horn relay? Its mounted on a plate that is behind the fuse box. The relay itself is up above the fuse box and is not really accessible where it is. The plate also has the control processor and another unit. I don't know for sure what the other unit is, so I'll call it the mystery unit. The plate has two bolts with 10 MM nuts on it. One is up in the dash, the other is down on the clutch pedal bracket. Both are accessible if you are laying on the floor with your head under the dash. I am attaching a picture of the plate.

I spent a lot of time trying to figure where the relay was. Then some more trying to figure out how to deal with the relay. Taking the plate off is not that hard. Spent longer poking around with a mirror and flashlight. Just do it.
I unplugged and removed the stop light checker relay to make a little room.
I unplugged the control processor unit and the mystery unit.
Loosened the lower nut that is attached to the clutch bracket but did not remove it.
Loosened and removed the upper nut. It will take a long extension but its a straight shot up to the nut.
Removed the lower nut.
Lowered the plate down and out from under dash and unplugged the horn relay. That left it as shown in the picture. There was still one more electrical connector plugged into the control processor. I did not unplug it.
I had ordered an Airtex 1R1779 horn relay from rockauto. It is a direct replacement. 10MM nut holds the relay to the plate.
Reinstall was the reverse. Plugged the new horn relay in and worked the plate back into place under the dash. Put the lower nut on first but did not tighten it. For the upper nut I put a very small piece of clear tape on the outside of the socket to hold the nut with the socket. The nut easily threaded onto the stud under the dash with the socket on the extension. Once I had a few threads under the nut I could pull the socket off, remove the tape, and then tighten the nut down. Tightened the lower nut, and plugged in the mystery unit and control processor. Put the stop light checker relay back on and plugged it in.
Entire process probably took less than an hour once I finally decided to do it that way.

As for background on me, I've been driving RX7s as my daily driver continuously since my first RX7 in 1982. I've spent a little time under the dash of these things. Its not as easy at 55 as it is was at 25.
Eric