Electrical problems like this are best found with a logical systematic troubleshooting approach. That circuit only feeds a few things. The stock headlight and associated switches, and the starter switch, starter relay. The headlight power is routed through the starter switch so that it can be shut off while the starter is engaged. So then the question is does it only blow the fuse when the starter button is pushed? If so the problem is in the starter switch, starter relay, or the wiring between them. If not then it is in the headlight side of the circuit, which includes the starter switch.
Ok so lets try to isolate where the short is. So get a bunch of spare fuses and your wiring diagram. ( If you don't have a shop manual you should get one.)To start I'm going to assume that every time you push the starter button you blow a fuse. Start by unhooking the wire from the starter switch to the starter relay, at the relay end. Make sure the end of the wire isn't touching anything and push the starter button. Still blow a fuse? If no the problem is in the relay. If yes, reconnect the wire to the relay, and unhook the starter switch end. Push the starter button. Still blow a fuse? If no, the problem is in that wire, If yes the problem is in the switch assembly.
Now I will assume that the short is in the headlight side. Every time you turn on the key you blow a fuse, or most of the time you blow a fuse. The headlight is powered by a wire from the fuse to the starter switch. From the starter switch it goes to the headlight dimmer switch, where it splits to high or low beam depending on the switch position. You aren't using the headlight anyway so unplug it where it exits the starter switch. Did that fix it? Yes well you aren't using that part so just leave it unplugged. No Well the next step is to unplug the starter switch. Still blowing the fuse? No then the starter switch is the problem. Yes, then the short is in the wire between the switch and the fuse. Are there any joints/connections in that wire. I suspect there is one under the seat. If so keep working back till you isolate what section of wire the short is in.
I suspect that you will find the problem is in the starter switch, but unless you like wasting money replacing good parts you need to prove it.
hope this helps
"How much a dunce
That has been set to roam
Excels a dunce
That has stayed at home."