Brake lights do not turn on

My taillights have three bulbs with multiple filaments; the hazard switch does operate and causes one lamp to illuminate and flash at each taillight (I believe it's the middle bulb flashing). Until I read some of the comments posted here I was unaware the brake lights illuminate the 2 sets of bulbs other than the backup lamp. So, Neither of my bulbs are getting lit when I brake. I'll start at the brake switch after I've removed an old trailer harness. Yesterday morning I went through an automatic car wash and suspect it got a blast of water that caused this.

That brake switch is unreachable where its mounted and I need to see where the wires connect but can't determine where they terminate. Does anyone have specific info? Even Chiltons and Haynes manuals are vague and non -specific.

I've had both taillights disassembled from lenses and viewable from my cab. And I disconnected my trailer harness and used a test probe into that receptacle while my pedal was held down -- no brake signal comes on. Of course when I test for hazard flashers in that same receptacle it indicates 2 energized flashing filaments (for both left and right lenses). So, there's nothing wrong with my trailer harness wiring.
I'm either not getting a signal from my brake switch, or my turn signal switch in my steering column needs replacement from what I have read of other's resolutions.
Or, maybe I just need to unplug and replug the computer module like I had to do to a my 2007 Malibu a few years ago when it began turning on and keeping on the brake lights intermittently while being driven. I wrestled a new brake switch under that thing and it fixed nothing.
Are there online sources where I can view specific info on this vehicle's wiring cabling plugs diagram layout and pinout of wires? I'd don't look forward to disassembling the dash interior ac duct without knowing the exact cause.

I'm waiting for a 1997 Chevy GMC Pickup C/K Truck Shop Service Repair Manual (CD Engine Electrical OEM) to arrive in my mail for detail info how to disassemble the lower dash and access the wiring terminals so I can check an input and output signal of that brake pedal switch. Hopefully, it does not require a combination switch on the steering column. I will know how to troubleshoot that soon enough if I need to. I rewired my trailer harness with newer connectors and new ground terminals in case it was what caused this conundrum.
How often have others sought this advice about failed brake lights and said it fixed the problem by changing the brake pedal switch? I'm getting no displayed diagnostic code faults which seems pretty useless for a modern vehicle that should indicate something wrong when there is not any illumination of the brake lights.

I haven't checked this GMC's 3rd brake light atop the cab.
Wouldn't any one burnt bulb cause all to fail? That would be the easiest fix ever.

I HAD a problem with my 97 GMC brake lights and HAD extremely, severe difficulty just getting a couple of my fingers to feel that connector of that pedal switch mounted above a shroud of sheet metal, braces, and steering column and had absolutely no room to grasp and disconnect a single connector nor even be able to put a test probe into any connector to check for voltage output. Now I feel like I need to wear a neck brace for 6 weeks. Whatever I did - touched or pushed some connector somewhere up there, it brought functions back to my brake pedal switch.
Would completely removing the upper dash and instrument cluster give me more access to actually change this switch, or should the steering column be removed? Removing the lower dash and ac duct there didn't even begin to help. I'm just wondering why its impossible to find more specific information that's helpful to resolving this. Even that GM service manual didn't expound on the difficulty of accomplishing this task.