1 - Did you buy this car or build it yourself?
If you built it yourself, what was the last thing you did?
2 - The dimmer switch may have gone bad. There is a variable resistor built into the dimmer, and that is what varies the voltage to the instrument lights. Check the fuse and then it comes back to question #1.
If the fuse is OK, there is a possibility that one of the instruments (one of the aftermarket ones) was wired incorrectly caused the variable resistor to burn out. After the defect in the wiring was corrected, the instrument lights work, but do not dim because the variable resistor is bad.
Yes sir, extensive rewiring and changes everywhere... and I follow your thinking.
For many months, I ran without issues, which gives me confidence in the Instrument Cluster (IC) not being root cause. More recently however, I replaced the wideband O2 gauge and "intended" to address a flickering in the stereo power. Next, I installed an aftermarket brake light blinker. So, I could be dirty on any of these 3 changes. In hindsight, I should have tested more thoroughly after each change. Overconfident, aging fool that I am. After the symptoms began, I also found the #4 fuse burned-out.
So, I began the process of Backing out my changes. Started with the Rear brake light blinker. Then I rewired center console gauges, stereo receiver, 12v lighter receptacle, etc. I removed the center console to better organize and simplify the configuration and have it look Pro. I'm confident this is fine now as I only have 3 connectors which are currently detached; Power, key-on, lamps on. All grounds come to a common post.
I'm keen to your sentence: If the fuse is OK, there is a possibility that one of the instruments (one of the aftermarket ones) was wired incorrectly caused the variable resistor to burn out. After the defect in the wiring was corrected, the instrument lights work, but do not dim because the variable resistor is bad.
I suspect this summarizes my issue. In fact, I ordered a new dimmer earlier today and it arrives tomorrow. When I get time to install it, I'll continue to leave the center console detached for a short time to ensure that extended use, with and without dash lights, are problem free. Then I can reattach console, get similar usage in AGAIN. And finally, if I still have the interest, I'll reattach the rear blinker taillights.
It's been many months since I've requested help, but as usual, you're like a Timex... "You take a lickin and keep on Tickin'"