The headlights and high beams stopped working?

Hello, sounds like the dimmer/High-Low/Flash to Pass switch (multifunction switch) is bad. I'll post the OEM diagram for you, it's a really simple circuit. Careful with the airbag circuits, I'll look up the service info on that, usually you have to wait for a few minutes with the battery disconnected to discharge any capacitors in the airbag system.

Being a 1999, with an OBD2 system you can pull trouble codes from the SRS system with a capable scan tool. Not just a code reader that reads engine codes. It needs to be one that can access the other modules in the vehicle.

https://www.2carpros.com/articles/how-to-gather-airbag-safety-system-codes

https://www.2carpros.com/articles/air-bag-removal-steering-wheel

No, you don't want to run any power to the yellow wire, notice the G201 (Ground), the head lamps Ground through, first the Under-hood Accessory Wiring Junction Block, 2nd to the Headlamp Dimmer switch and then the black wire runs to G201. So, if you want to verify the Headlamp Dimmer switch is bad, Ill post a diagram of where to run a wire, you just need to bypass the Dimmer switch with a wire but let me post a diagram so you're sure of what to do.
As for the airbag system, were you getting changing live data from the airbag system?

Hopefully this is easy to understand. The left side bypassing the Dimmer switch will turn on the Low Beam using the Headlamp Switch because the Dimmer switch is at rest in that position and the circuit Grounds through the Headlamp Switch. And the right-side bypass with turn and keep on the High beam. Jumping power to the yellow wire will send power directly to Ground, so don't do that. The Headlamp Switch and Dimmer Switch are both on the Ground side of the Headlamp circuits. Let me know if you have any questions.

Well, that's not much data. Here's the SIR info anyway, how to disable it first and then the wiring diagram for the system. If you eventually want to replace that passenger side module.

I hear that, I've found over the past 20 years that almost every job ends up being more than I had planned, or I find something else that needs to be repaired. It's just something that needs to be pretty much assumed when you start getting into things. There are a lot of people that need to keep their older vehicles on the road, and I understand that. Who can afford some of these car payments, they're really ridiculous. Or even just shop prices alone, I try to cut my customers a break on that, Ivee seen some dealerships charging up 200/hr. But anyway, here's the service info on the B1000 if you happen to need it. It looks like a B1000 can be either the Cluster failure or ECU failure, but the ECU failure is not the PCM, it looks like the SIR module failure, where it will set the airbag light on. Although the BCM can set "B" codes, I don't see a B1000 listed in the BCM trouble codes. The SIR sensing module is under the passenger front seat. I always disconnect the battery and wait at least 15min if unplugging any airbag components, service info states a 10min wait. And most of the time the modules under the seat are grounded through the bolt that secures them to the floor and can set off the airbag if that module is unbolted with the battery still hooked up, I've seen someone do that before.