Where do you start?

I would go buy a test light, the one that lights up when you touch ot to 12+
Most auto parts stores and tool shops will have them. From there you should get a good wiring diagram...I'll attach one to the end of this post. Then start tracing the wires, It can be frustrating but well worth the effort.
At least your horn isn't going off when you step on the brakes.
This is a 66-67 diagram

66 only

I'm not sure what year your car is, if it's not one of these i'll post the correct one for you.
Geoff.