Okay I am assuming a 1980 is a first Generation with the 2 ignition coils (both generation are basically wired the same they just look different)
First the high and low beam headlights and ignition aren't fused so they are not relevant
Coincidentally
the Horn
The directional signal
the brake lights
are fused all together on the same fuse
So If the brake lights are out also
Chances are the following
The fuse is blown or the connection on the back of the fuse is pulled off
The fuse isn't making good continuity in the holder
The wire from the fuse to the directional flasher is bad
The wire from connector #19 to the fuse holder is bad
This can all be checked very quickly with a volt meter
No guessing required
Off course this theory doesn't mater if a Previous owner has changed things