OK, before everyone chimes in and says "that's the well-known and well-documented Mach radio display problem," let me say that my Explorer is a 1996 model and does NOT have a Mach radio.
During the day, when my lights are off, the radio display works fine, as it always has. The other day, I was driving in the early morning and had my lights on. The radio display was very dim, barely visible. I couldn't get it to change by turning my dash lights up or down. A little while later, it was working fine again (nice and bright). Last night, it was dim again, and I never noticed it go back to normal.
I pulled the radio out and did not see any loose connections.
Any ideas?