Instrument cluster questions

On my 64 Dart, the fuel and temp gauges do not work and the lights are super dim and wanted to replace them anyway, so, I pulled my instrument cluster to take a look at the back. Pics below. This thing is a mess......look at all those solder and corrosion.
What should my next steps be here?
I've also read that maybe the voltage regulator may be shot and if I understand right, that is built into the fuel gauge??
Oh, also, what is that plug to the left of the speedo cable? It wasn't plugged into anything when I pulled the cluster out and I do not see a spot for it within range of the wire either?? Previous owner shenanigans?

Thank you all!

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