Study the schematic. Get a test light and start at the battery terminal, grounding your test light on the chassis at all times. Do not use the battery ground lug. Work outward, following the red 10 and 12 ga wires. Test each point where the red wire bifurcates into a new branch circuit. Just follow the red wire...like wizard of Oz and the yellow brick road.
With a failure this complete, you will quickly find the problem by following the flow of current until it disappears. Test for power at the distribution lug, the horn relay, the alternator red lug, the bulkhead connector, etc. Just follow the red wire. Faster and easier by far than fumble fixing and/or guessing.