Rear defrost grid switch wiring
 

I'm hoping someone can help me out. I've found a rear defrost grid switch to install in my 71 GTO. What wires plug into where on the switch? The switch has a plug on the harness that looks like it would take three wires and another separate wire.

On the car side I have a spare blue wire coming from a relay looking thing that is mounted on the rear of the instrument cluster, at the top outboard side. It has two wires attached. I don't know where this blue wire goes or for that matter - what that thing is or does.

Then I have a few possible wires that could connect to the defrost switch. Mostly black or brown wires.

I know this is kind of vague but I'm open to any suggestions.

Thanks in advance
Glen.