If the horns make some noise and the connections are tight, I would say it's the horns themselves that are at fault. I've heard of people spraying them out with WD-40 to get them to work again, but haven't tried that myself.
Amp light is probably a failed rectifier in the alternator.
Remove the switch for the brake warning light from the junction block where all the hard lines meet, and screw it back in. It will trip anytime you bleed the brakes, and that's how to reset it.