Headlamp & Horn

This is likely something like a poor column ground. No the horn does not go through the headlight switch. If you are going to do much in the way of electrical you are going to HAVE to learn a bit about diagrams I admit, the 73/ later diagrams can be a PITA. I use them in combo with the simplified 2 page ones at MyMopar. Those are not detailed as per OEM and leave some things out. But they DO provide a line drawing that can be easier to follow

I would start by accessing the horn relay. The horn button is normally small black. Pull that off or back probe the connector and see if the voltage goes to ground when operating the horn button. Then turn on lights and check same. Also try grounding that terminal with a jumper wire and see if symptoms change