Spent some time this morning looking at the wiring diagrams and it still isn't making sense to me.
One of the tests I did last night was to bypass the ignition switch again and ensure that there was power to +X, +15, and +54. I didn't also include +S because that is supposed to only be for the key and seatbelt warnings. According to the FSM, that should represent the key in the "drive" position:
I also verified that when the key is turned to "drive", I get battery voltage on pins 15, 54, and X at the switch. I didn't attempt a voltage drop test on those, but everything behaves the same way when I have the wires clamped together with a hemostat, so I'm assuming that the switch is working okay there.
When I do that, I get the fuel sender tickling and the low fuel light on, the coolant temp needle moves a bit, and the lights come on for Check Engine, oil level, charging, parking brake, and ABS. The Brake Fluid light doesn't come on and the pump doesn't run, even though the pedal is hard as a rock.
If I either pull the +15 wire from the clamped mess or turn the key partway between "Parked" and "Drive", the cluster is still and dark except for the charge light and the Brake Fluid light. The pump will run and will then shut itself off. If I leave it like that and pump the brakes a few times, the pump will kick back on and then turn itself off. So it seems like the power supply and the relay feeding the brake pump is working correctly at that point and the pressure switch is working correctly. But, if you look at the wiring diagram, the brake fluid light - and the pressure switch - should be getting their power from the +15 circuit which is not showing any voltage at the ignition switch:
#294 is the pressure switch and #293 is the relay for the pump motor. The control circuit pin 86 comes from +54 off the ignition switch and is grounded through the pressure switch which should be getting it's +12 through the Brake Fluid light off of +15.
This is where my lack of understanding about how electrons move about the cabin makes me start to stare out the window aimlessly and drool a bit. If +15 goes straight from the ignition switch to the gauge cluster - which it appears to do - then that light shouldn't be on when +15 is unplugged.