Starting to clutch at straws here now.

The test you did earlier was to test the brake fluid reservoir level according to the 3 pin connector.

I haven't taken a brake fluid reservoir cap apart to see exactly how many sets of switches there are inside, so let's see if the 2 pin connector works on different contacts within the cap.

Remove the 2 pin connector and join the wiring loom terminals together with a short piece of wire. Fingers crossed the light goes out.

ETA : Thinking about it, it should be OK as you've got continuity between ECU terminals 9 & 10