It's probably electromagnetic. Thus when you throw the switch, it should power the magnet (which you can think of as a switch that can handle more power) closing the circuit that allows the higher current to pass to the device.

If I'm right, there should be one terminal of the relay that goes battery->switch->relay with the next terminal going to ground. This should be basically the low amperage side of the relay whose job is merely to throw the "larger switch" inside the relay allowing current to pass to the device. So when the low side is closed (the switch on your dash is thrown, or turned on) it should close the other two terminals so your test light lights up or your ohm meter goes to zero when connected between those two.

The high side of your relay should be wired battery->relay and the other terminal relay->device the other terminal on the device (in this case the upper plug on your diff) should go to ground.

Your mission; should you choose to accept it, is to figure out which two terminals control the low side. Once you've done that, the high side should be a piece of cake.

I deliberately left out the fuse part to simplify the explanation. Make sure you put them on both wires coming from the batteries. It's always a good idea to install them close to the battery.

 

Well - I chose a different mission. My gut feeling is that I had a bad relay or I shorted it during the install so, I wired directly and left the %^&$ relay out of the equasion AND IT WORKS!!!

I am using a 4A fuse, but there is no load. What amp fuse should I be using?

And a great big THANK YOU ALL for your help and advise. You have been awesome.

 

Woo Hoo! Congratulations! I have no idea as to load. I put a 20 amp on mine, but whatever works. The whole need for a fuse is incase something shorts it'll protect your battery and whatever other system that chooses to get involved from being smoked.

 

3-4 amp draw from what another member told me when I was asking about installing mine.
So glad the thing is working for you.