Why does my brake fluid light on dash stay on all the time?

I have a ford focus 2002 1.8 diesel.

I noticed that the brake light stayed on whilst the handbrake was off. I checked the fluid levels and they are still on max.

I changed the brake shoes on the rear and the pads on the front and it still stays on.

The plug is a little corroded so I cleaned it out the best I can, I checked that there is still a feed going through the plug and there is.

I also checked that the switch on the handbrake has a feed, this too has a feed. (I am wondering if there should be a click when pressed?)

I dont know what else to check or do, apart from have the dash checked but that is expensive.

Any suggestions on what I can try or if anyone knows what is actually happens will help massively as I need it MOT'ed ASAP!

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