The yellow wire should be at the headlight switch in the dash. On my '96, it's also the wire that runs to the DRL module and provides signal for the module to operate the DRL relay.
The thing I liked about the store bought harness method is that you can always just unplug it and put the truck back to normal. I went with an LMC harness since it was only $30, but I put in my own relays and relay receptacles, some quality headlight connectors, and added the diode. $30 for a harness DIY starter kit wasn't bad at all. You like the Hella relays, but I went with Bosch. Either one is good.
https://www.bosch-automotive-catalog.com/en/product-detail/-/product/0332209150
https://www.bosch-automotive-catalog.com/en/product-detail/-/product/0332019150
I notice the headlight switch comes before the multi function switch in diagrams. Dumb question: Does the original yellow wire need to be kept intact (tapped into) in order for the multi-switch (high beam selector) or anything else to stay functional? If so, I'll want to get one of the better alternatives to those nasty Scotchlok connectors. My '89 doesn't doesn't DRLs of course.
Or would I just cut it and run my new wire from whichever end of the yellow is hot when the headlight switch is turned on?
Perhaps if I look behind the switch at its wiring harness I'd know more. And obviously I could cut it and test all of this but that wouldn't be ideal if it turned out I shouldn't have cut it. Just wanting to know what to expect first.
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Just did an idling voltage drop test on the low beams since I can reach the backside of their connectors without removing anything. I tested the total circuit at two locations (at both lights.) The passenger's side reveals about 2.1 volts lost, using of course the battery. The driver's side was a little less, about 1.8 volts lost, using a power point I'd installed on the firewall that has a direct line to the battery with 10ga wire.
So calling this a 2v drop, I'm losing about 40% of my light output according to various Internet Facts™ (14v lumens output "1356" versus 12v lumens output "803" for a halogen). The truth is probably somewhere in between that amount of loss.