Emergency brake light is on and stays on

The van (iveco daily, sorry don’t know the type anymore) of my father has a brake warning light on. Once I removed te cover I saw it was a single wire that was disconnected from the switch. I thought ok easy fix but when I connected it back the light was still there. Also a strange thing was … when the switch is open I only have something like 3 volts which seems very odd. Could it be that the brake fluid sensor is faulty? I don’t have any wiring diagrams so not sure if those are in series. However 3volts is odd maybe a break in the + cable?

So to conclude the wire was not connected and the brake warning light was on. Once I connected it it was still on no matter if I pressed the switch or not. If I push the sensor (open circuit) I measure only 3volts.

A subquestion (sorry for that) is how does the signal of a brake fluid sensor actually looks like? Is it also like switch or does the sensor acts more like a variable resistor? The sensor is a two wire sensor. And can I bypass it with a small resistor to check if the sensor is faulty?

Thanks in advance