Here is how it works:
As Wayne said, the light in the switch is powered by the ICM, it's not part of the actual circuit involved in locking or unlocking the doors, tho it does share a ground with the switch, so if the backlight comes on, we can probably assume that pin 4 has a good ground.
With the button at rest, there will be voltage on pin 8. I don't know if this would be 5 volts, 8 volts or battery voltage of 12-15 but I'd bet it is 5 volts.
When you UNLOCK this voltage must go to ground, that is, zero or very near zero. This tells the BCM you pressed UNLOCK.
When you press LOCK, the voltage at pin 8 drops from (what I think is, again, 5 volts) to some value lower, probably about 2.5 volts. THis is due to the resistor.
This tells the BCM you want to lock the doors.
So, do you have voltage at the BCM? If not, cut the wire. Do you have voltage now? If so, the line to the switch is shorted to ground somewhere, or the switch is in UNLOCK constantly, grounding the line.
If you have voltage at the BCM but not the switch, then the wire from BCM to switch is open somewhere.
I'd measure the voltage at the BCM connector C3, pin 49. See what happens to the voltage when you press LOCK and UNLOCK. You should get the results I gave you above.
If pin 49 is at zero all the time, the wire may be nicked and shorted to ground somewhere.
If pin 49 has voltage and it never changes when you press the button, the wire may cut somewhere.
IN all cases, make sure your meter is grounded well, so you can trust the readings. I like to connect my meter ground first, then measure some known point, like the battery positive terminal, if I see my 12-14 volts of battery, I know I"m good to proceed.
