The more I think about this mod the more I think the best way to do it on a truck which is not currently malfunctioning in any way would be to rip open the harness on the driver's side next to the battery, and splice in there, then close it back up.
There are two wires in the harness to interrupt on that side. This would require only a couple of feet of wire, relays in sockets, and one or more fuses in sockets or soldering in a fusible link depending on how cheap one is.
The ultra-cheap could even skip the sockets, but why? You'd save pennies. Only reason would be if you had everything else just lying around.
There's even a ground screw right over there in front of the battery already. I'd put the relays to the left of the battery though (outboard, that is) rather than in front of it, just because that's an easier place to fiddle with the harness and you're sure to be upstream of the splice between the two headlights, which is in the harness anyway.
I personally ripped all that wiring out, and most of the wiring to the low vac sensor, which I relocated to the rear of the fuse/relay box when I relocated my vac can to below the brake booster servo. Since I had bought the LMC kit, I integrated that wiring into my harness, and abandoned the original headlight wiring and connectors entirely. But that stuff isn't really a problem, unless it's actually been damaged. Tapping into the harness where I suggest will ensure that you're running through very little wire before the splice. It would be cleaner to rip out everything up to the splice (again, the splice carries power to the right headlight from the feed wire, there's two splices, one per beam) but that requires more harness spelunking.
