Ground connections can be dicey which is why manufacturers stick multiple ground straps all over the place. Some of them evenutally corrode to a point where they are passing some voltage but not enough to power what they're hooked up to. You see it a lot in cars with one dim headlight. Jiggle the connector to clean off the corrosion and it brightens up.

I'd start by checking all the fuses and connecting blocks. Take them apart, give them a shot of electrical contact cleaner and reassemble.
Check your main grounds to the frame and engine. If they are loose or corroded looking, take them off clean up the connector and the spot where it lands and bolt them up again. Same with the relays (in the box under the hood). I had a situation where my headlights would cut out and it turned out to be the connecting block under the dash where the interior wire harness met the harness in the engine compartment. The connection was just weak enough that the headlight relay would drop out. By eye it was clean, but it didn't work until I sprayed it with the contact cleaner and ran the plug in and out a few times. As for that horn it's either oversized or you have a dead short. Check the harness on the steering column for worn through insulation or melted wire.