Here is a gen1 wiring diagram for you: https://www.flickr.com/photos/wandereye/2274847735
I would disconnect the 9 wire connector from the light switch to the main harness. May have to remove the tank to get to it.
To see if the signal problem is related to the signals, the blinker or the switch, I would do the following on the harness side of the connector:
Confirm that the Blue/White wire has 12 volts with the ignition on. It is just a 12 source for this test.
Jump from Blue/White to Gray to prove if the right signals come on solid without the blinker.
Make the same jump to Green to prove the left side signals.
Confirm that the Orange has 12 volts from the turn signal relay (BLINKER) with the ignition on.
Jump from Orange to Gray to cause the right signals to blink.
Make the same jump to Green to cause the left signals to blink.
The horn is wired hot on the brown wire to it from the ignition switch and the horn switch/button provides ground to power the horn. To see if the problem is related to the switch or the horn:
At the same harness connector first test for 12 volts through the horn to the Black.
Then ground the Black to see if the horn works.
When you find the problem areas, check the wires as well as the devices. The switches may just be corroded.