Fuel switch relay

[Captain J]

I’ve 1987 GMC Sierra R3500, that I just restored, I had some issues with fuel electrical wires.
Nothing worked, but I fixed most of it, I installed new tanks, new sending units, wire harness is good.
But: I need your help in finding what’s missing here (in the pictures)?
I need to know the relay/or whatever missing (if any) that’s located behind the instrument cluster, for the fuel switch selector in the dash.
I checked and chase all wires, from the fuel selector to the fuel selector valve (underneath the passenger side), to the fuel sender units, they all ok. I ended up with the selector fuel switch’s 4 wires, 2 go to the fuel selector valve (gray & tan), and 1 ground black and white, and 1 tan that enters the bulk head behind the fuse box, then it end up with 3 wire plug (black and white, red and black, tan and white).
If I excited it with direct 12v from the battrey, the fuel switch will work on both fuel pumps.
So it’s missing a part, does anyone know the relay type or whatever missing name?
Pictures attached.
Sorry for the description, not an expert in terminology.