Try to disconnect the horns. There are two at the front passenger side of the bumper. Then try the horn to see if it blow the fuse. I had mine get packed with mud and burn out. They would blow the fuse if I hit the horn. If the fuse blows with them unhooked, then you have a shorted ouot wire on the way to the horns, if not your horns are burnt. Speedo could also be a groung issue on the back side of your instument cluster. I ghaven't had my Z a part but on a GM they used a ribbon into the cluster and the ground indented itsself a bit and made a poor conection causing similar problems. I think that the seats might be on the horn fuse as well. Pull the plug a part under your seat and with a test light or multi meter check to see if you have power into the plug.