Hello and welcome to justanswer.
First thing you will want to do is to make sure the ambient light sensor in the center of the dash (on top) is not covered or obstructed. Then make sure the sensor is in place, you should see a small plastic dome shaped sensor. If you do not see it, then it has fallen into the dash. Let me know what you find there and then we can continue with diagnosis.
No sign of an ambient light sensor on top of the dash and there is no hole for it. Should it come up through the vent near the windshield, or where should it be? I think the problem may have started after I cleaned the windshield and dash board with a towel, so it's possible I've pushed something out of place, but I can't see what or where.
The ambient light sensor is located up in the center of the defrost grille area.
The upper dash panel has to be removed to service this sensor.
Remove the end covers and 2 phllips screws on each end of the dash.
Remove the passenger side dash vents by prying them out of the dash. Use a flatbladed screwdriver with the tip wrapped in tape to keep from scratching the dash. Remove the screws behind each vent, then reach through the vent opening and depress the retainer tabs that hold the grab handle into the dash. Pull back on the handle while pressing the release tabs.
Shift the shifter all the way to Low 1 range and remove the gauge cluster bezel by pulling straight back. Then remove the screws around the perimeter behind the bezel.
Then the upper dash panel can be lifted and pulled straight back. Reach underneath to access the ambient light sensor. It fits into the hole in the dash and secures with a 1/4 turn.
Thanks, XXXXX XXXXX makes it clear where the sensors are. I was looking for one central sensor with clear plastic and was confused by the two black plastic domes which I thought were clips or spigots holding the grille in position. I confirm that both the ambient light sensor and sun load sensor are in position and not covered up. So what's next?
You will need to test voltage output from the ambient light sensor. If you have a scan tool like the dealers, you can read the voltage from the scan tool. Since you do not, read the voltage at the BCM connector. You will leave everything plugged in, just use the voltmeter probes to insert into the back side of the BCM connector to read voltage. The BCM is located under the drivers side of the dash, remove the panel under the steering column for access.
Measure voltage at the purple connector, pin A4 gray wire, and pin A6 white wire as shown in the diagram below. With the key on and daylight hitting the sensor, you should have below 1.3 volts. Cover the sensor and you should have above 2.2 volts. If these readings are incorrect, replace the ambient light sensor. If the readings are correct, replace the BCM.
Thanks, I'll let you know what I find after checking the voltage later today.