Valid conclusions, plus a few more thoughts for you here

Seemingly the new sensor behaves better during testing than the old one, so definitely go with that one until everything is working, then you can go back and re-test the old one to see if it still works.

|When I connected the external battery to pin 54 and the light came on in the NEW sensor, what does that tell me? That power is now able to traverse the sensor, run through the wiring at pin S and light the high mount bulb. BUT when I just put everything back together and hit the pedal, the high mount bulb does not light.|

That tells me your new sensor works just fine and that when you put everything back together you are no longer getting power to pin 54 when the brake pedal is depressed. Either the brake switch is faulty or you didn't have the car battery re-connected or you have a contact issue in the socket.

Are you sure the other brake lights are still working normally as you earlier stated? Apart from a few possible mis-steps during testing, it's only the 3rd brake light misbehaving? When you're sitting in the car doing testing it's not always easy to know if the main brake lights are illuminating unless the rear of the car is close to a wall. I'll sometimes keep a board propped up against each tail light so I can see a reflection.

I have completely discounted the brake switch from your issues as you said the other brake lights have been working normally and I believe some of your testing found voltage at 54, 54R or 54L, just nothing on S and the 3rd light only illuminates when you apply external voltage either directly to the base of pin S or to pin 54 of the new sensor. I will mention that the brake switches in the '94-'95 940s can be problematic at this age. As they wear, operation can become intermittent. They're a different design than the earlier barrel shaped ones and frankly a major, major pain to remove and install. Although a cleaning of the internal contacts can get you a bit more life, both mine finally needed replacement in the past couple of years. You can meter test the brake switch in place using aligator clipped test wires, but be cautioned that an intermittently failing switch can look good during manhandling and testing.

You're testing also suggests the Yellow-Black wire for pin S is good to the rear. However, the inconsistent test results are suggesting you may have a contact issue in the relay tray socket. Inspect the integrity of the socket more closely from above and below, with particular attention to the pin 54 and S locations. Use spray contact cleaner (not WD-40). If it turns out one of the contact sleeves has expanded due to heat or physical effects then you can try to carefully close it up a bit, but save that thought until you better identify the issue.

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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now