Regarding the breaklights, if the turn signals work then the ground should be fine (same filament). I would look to the switch or the turn signal switch may be killing the stop lights as baldy said. You might try jiggling the turn signal handle with the brake depressed while a victom watches for glimpes of light.
The horn does sound like a ground or a bad horn to me. You can make temporary grounds to test this theory. Just give it a new ground using a clip jumper. If you don't have one, make one as this is a great diagnostic tool to have in a shop (12gauge wire with clips at either end). Just clip one end to the ground on the horn, and the other to a good clean metal contact. If it is still weak (thumps) then see if the power side is good enough with the same manual jumper (battery to pos side of course). If it works then, you'll have to look to the wiring inbetween somewhere that is weak.
