[VetteVet]

Well I think I can. LOL.
When the factory installed the light cluster they needed some way to test the warning light bulbs in the cluster. The wires on 5 and 2 are for the sending units on the temperature lights Pin 5, and the brake warning light, pin 2. The sending units are variable resistors to ground that will ground the warning lights when the sender resistor reaches a low enough resistance for the light to ground and turn on. In the case of the temperature sender, it grounds when the engine overheats.

The brake warning light is grounded by the switch in the brake combining valve by the master cylinder, when the brake pressure becomes unequal between the front and rear brake systems, usually caused by loss of fluid in one of the systems.

The extra wire in 2 and 5 are just bypasses direct to ground to turn on the lights to check them. This is done when the key switch is turned to the start position and the wires are grounded by the key switch through a separate connection in the switch.

Most of the harnesses are wired with the two wires at the key switch instead of the cluster plug but the effect is the same. If you were to turn on the key switch and ground the wires at the sender the lights should come on.

My 'question for you is--- have you checked the new cluster to match the pins where the plug goes with the copper strips to the gauges. I think you are aware that the aftermarket sensors have to match the gauges that they go to. If it's like the normal conversion from stock light clusters to gauge clusters that many of us have done then all you need to do is change the cluster plug wiring around and change the temperature gauge sender, or in your case the aftermarket senders. For instance the oil sender wire on pin 1 may go somewhere else if you have an electrical sender instead of the mechanical tube that the gauge cluster uses.
Another example is what about the battery gauge or are you getting a voltmeter?
This would change the wiring to pins 1 and 12 on the plug.