65 Dart. Brake lights and dome light stay ON....Why ??

Check that power goes in and not out till the switch is engaged by the pedal. If it's constant hot at both sides..the switch is bad.
The dome light could be the jamb switch.
You would have running lights only if the turn switch wasn't working, could be stuck on with it working incorrectly. How's the headlight switch? Have you checked it, volt meter to see if it's switching correctly.
The horn has a hot lead...grounds to activate. I had a rim blow horn stay on...the ground wire insulation was worn. Check the one ground wire for the horn, under the horn ring.pop it off.