How do you take dash cover off on a 1990 Buick Riviera?

The 1991 Riviera and Reatta had power supply modules which we often had problems with at the dealership when they were still under warranty. Sometimes you could tap on them and make the problem come and go to diagnose.

The automatic light sensor is a fish eye bubble on top of the dash.

DO NOT use dash mat,it may cover the sensor and disable the auto brightness feature.

On the right side of dash. Do not use #1228253 they will give trouble.

The fuse block is located under the center of the dash. On the left front
side of the console. I believe you're looking for fuse #2 20amp.
Hope this helps.

Could be. Check the wiring at the oil pressure sending unit. If the wiring is good, replace the sensor. It is located Under hood, passenger side, rear engine area, above oil filter, mounted on lower timing cover. Curtorsey: Auto Zone.

sounds like a bad ground in the wiring

WILL NEED A CENTRAL POWER SUPPLY MODULE TO FIX THIS...THERE WAS A VERY FEW CASES OF A BCM(BODY COMPUTER)CAUSING THIS BUT VERY UNLIKELY

Look on the dash. I believe it's on the passenger side over the glove compartment.
It's easy enought to test. Turn lights to automatic and cover it up. It's time delayed to prevent the
lights from coming on if driving under a bridge.

about $100, mostly labor on the dash illumination issue.

The light sensor is in the dash board top about 2/3rds of the way over toward the passenger side. You will see 3 or 4 small slits, and that is where it picks up the light, or absense of light. Is the twilight control switch, located under the main light switch, in the position to turn the lights on automatically? It is a slide type switch, and must be slid over to the right for the automatic control to work.