Bench Testing

I didn't test lights and illumination. I guess I had no concerns there other than bulbs and bulb holders.
I did test gages using all the methods recommended. First check for resistance in the gage. The used some D Cells in series to check response to 3 or 4.5 volts. Finally hooked up resistors to represent sending unit resistance, and then used a car battery to power the IVR and see how the gages responded to low, mid and high resistance.

https://www.forabodiesonly.com/mopar/threads/gauge-cluster-issues-ivr.455893/post-1972861238