How does the low brake fluid light work?

As the title says really... I have no brake light pop up when I pull my handbrake up on my 1990 90 after putting in the td5 dash and warning light panel, upon checking for the white and yellow cable at my handbrake it appears I don't even have a cable there. So I'm guessing I never had a handbrake warning light before... Errrrr.....

This prompted me to undo the plug on the brake fluid res and the warning lamp didn't come on either... The ignition was on but the engine wasn't.. I'm positive that the wiring to the dash is correct and present as I used my old loom and I've checked my wiring over and over again..

Is there a way I can test the bulb ie earth the black/white

Does it run to a relay somewhere?

Will it have anything to do with the oil pressure relay ticking as the oil light flicks on when warm (I know this is fooked rings but I was just wondering if it was a dodgy relay or something)

Thanks

switch will make the circuit to earth, so you need to join the wires afer pulling from the switch for light to come on

Ok I might pop go check that now with the ignition on.

There should be a test button on top of the cap. Push it in with the ignition on and the "((!)) BRAKE" light should come on. The handbrake switch (if fitted) grounds the yellow/white wire and lights the "(P)" warning light. "PARK BRAKE" is listed in the manual for mine as Australia only.

I earthed the double black/white cables to the truck and the brake warning light came on so I followed the earth lead and it goes to the same earth as the starter relay, it's a poor earth and the connection on the res side is ****e so I'll redo that and it should hopefully cure it. Another job ticked off....