firebane, they are under the steering column area to the right of the fuses, my horn wasnt working properly and i had to find it. Its a little module on a tiny little mounting plate mine was faced away from me. Theyre not the easiest to locate but are pretty easy to take out.
This (the headlight tweak) reminds me of a mod I had to do on my motorcycle. I installed a larger horn (Stebel nautilus - which I did to my square too and recommend it) because the little stock horn was not loud enough to warn others while on the highway.
I had to end up using a relay to divert power directly from the battery to the horn because it drew something like 4x the amperage of the stock horn. For the headlight mod, I assume you can use any 12v relay rated for the amps your headlights are rated at. I'd buy one with 10% more capacity than I need just to be safe. You can probably get em cheaper than the $24 each relays sold in the link above.
I am going to make my own xenon bulb adapters so I will have to do this to my square if I want to get enough power up there. Also going to install an aux. battery and maybe a second alternator with the ambulance dual alternator kit.
I used the headlight relay harness from LMC truck and it was an easy installation. I put an additional power junction point on the passenger side fenderwell (same type as the one on the firewall behind the brake booster) and ran a 10 gauge wire with a fusible link over to the battery positive. That gave me a good place to use for powering the headlight relays. Then I attached the relays to the radiator support near the battery and ran the cables to the headlights.
The harness I got had really bright yellow wire, which looked out of place, so I put it all inside black split wiring conduit so it matched the other wiring in the truck.
I went ahead and performed the Big 3 wire upgrade. I used 6awg battery cable from Prowire as audio battery cable is not as robust. It has helped the alternator keep up with the electrical loads at night. Not to mention my starter spinning up faster. I did run a 2/0awg to my starter, but it didn't do as much as the Big 3.
I use relays for other items on the truck, but not the headlights.