'90 Signature town car - dash lights fade in daylight - where is the sensor?

Not sure, you could explore. Take a sheet of thick copy paper, maybe two of them sandwiched together and slide it around in the daylight to see where the dash brightens, should also work if you park in garage and use a flashlight to see when the light hits the sensor and dims the dash! Hope this helps

It may be on top of your dash?