will not kick in viscous coupler?

There are 3 fuses that run the system I would check first: CLUSTER A and MODULE 1 in the fuse panel inside the car, and 4WD fuse in the ER fuse panel.

This guide can help you check the fuses:
https://www.2carpros.com/articles/how-to-check-a-car-fuse

Also, I would run the codes to see if any are stored.

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

If there are no codes and the fuses are okay here is how to change the clutch control motor for the coupler and how to reset the operation of the motor as well. Also, I have included the wiring diagrams for the system so you can see how it works. Check out the images (below). Please upload pictures or videos in your response of the problem so we can see what's going on.

Should there be 12 v to the 3-pin connector at rear differential? I only have 5 v.

Do you mean to the electronic magnet valve clutch? If so, it looks like it can be a 5-volt sensor system much like the engine sensors. I would do a CAN scan to see what codes the system has. You can get a CAN scanner (Controller Area Network) which will work on most cars from Amazon.

Here is a video to show you how:
.

https://youtu.be/u-4syLc-ifQ

Also, did you check the fuses I mentioned above? Please let us know what happens.

anything?