Instrument Cluster Dimmer Nerfed by SW update?

Until this year the dimmer wheel on my 2004 Prius would allow me to turn the instrument cluster lights all the way off. Now it only dims a little bit then scrolling to the end of the wheel does not dim it further. What is irritating is that it doesn't go dim enough to eliminate all of the glare cast on the windshield any more. Was there a recent firmware or software update that might have been applied at the dealership during my last oil change?

I don't know when this started exactly but it must have happened this summer when the days started getting longer and my commute to work was no longer in the dark. I just discovered it this morning on my morning commute now that it is dark again.

 

Bisco,
Update: I've replaced the prius dash dimmer with two different ones from salvage. They both exhibit the same behavior. Do you have any other suggestions?
Thanks,
Eric

 

I wasn't able to figure out exactly what was what from the EWD, however I was able to compare the voltages / impedances on a working car of the same make and model to mine. I spliced in a wire to chassis where I thought the ground wire was but that didn't make any difference in observed operation. This is the test results pre splice for the working car:
NC NC
NC HI Z
NC Short / LOW Z
NC Short / LOW Z
NC NC
NC NC
NC Short / LOW Z
NC NC
On the car with the problem the bolded connection read a stead 33 ohms and that is the one I spliced to ground.
I also ran test with lights on car off, car on lights off, and car on lights on. When the lights were on and the car off the one above the bolded connection read 12V on the working car and 0V on the non working. On the Car on Lights on test that same pin read 14 V on the working and 1.8 on the broken. There were some other .01V differences on different pins and different tests but I believe that these are measurement error.
Patrick any further thoughts? Am I at the end of fixes that an noob can do? I think that there is something wrong with the hot line bases on the tests above. I haven't retested voltages since splicing in the ground.
Thanks,
Eric

PS. I also tested the working dash dimmer in the broken car and the salvage ones in the working car. The salvage ones work just fine in the working car as well as my original that I thought had broken.

 

Andrew,
The wire I grounded was white with black stripe. The NC stands for no connection meaning that there isn't a wire there. The 16 connections I listed correspond to the 16 pin connector on the rheostat. I wasn't able to figure out which one was officially pin 1 there were no apparent markings. The wire I grounded appears to be the 4th connector hole down on the right hand side as viewed with the connector unplugged from the back of the rheostat. Fortunately there are only 4 wires in the 16 pin connector. It is all the lights on the dash that are not dimming properly. I say properly because they dim a little then stop and do not go to full black as they used to.

 

I doubled checked the ground I added and it is grounded. The color of the 1.8V is red. The wire above that is blue and the bottom wire below the ground is grey.

 

I've only got 1 7.5 amp fuse the other two sockets are empty because no fog lights etc. Replaced the fuse even though it looked good, no change. Re-examined the wiring harness going to the rheostat and can't find any evidence of an additional wire blue or otherwise. My one blue wire does happen to be sky blue. I haven't had a chance to get the volt meter out again to recheck voltages. In the mean time any other suggestions?

 

Ah, got it. I removed fuse #17 (Panel 7.5A) and the dash lights will no longer dim at all. Thank you for all the help.So this probably means that somewhere on the red line there is some sort of very high resistance short to ground?