How to test to see sensors are working?

The band side of the diode is facing towards the sensor. There is my wiring. Upper right is my ground, middle is my multiplex negative output, and lower left is power(grey wire coming from Backup battery). The red wires coming from the wire loom are just the wires I used to extend the harness from the alarm brain.

That's how I connected the wires from the sensors.

The sensors do light up, but I've notice that when they are triggered, the voltage drop on the negative output wire only drops from 12v to 8v.

I triggered the hood pin switch and notice the voltage drop on the same wire was from 12v to 0v. I'm not positive if the voltage drop matters for the sensors. What voltage drop is needed for the alarm brain to trigger the alarm?

Here is how I wired the shock sensor - connected the green wire to the blue one.

Yes, 3 wires you've stated.

The thing is that the sensors do not ground the alarm(both shock and glass). There is only a voltage drop of of 8v from 12v on the neg trigger wire from the alarm.