[Valiantpatrol]

AllanC gave you the best/most accurate info. I think your radio is broken. Over the last decade I've bought a bunch of these from local yards. About 1 in 5 functions well in every way, with a working CD player. Approx 1 in 10 has the burned out lighting, both display and back lighting. Approx 2 in 10 has a staticy/intermittent volume control. The vast majority everything works except the CD player. I'd like to find replacement motors that spin the CD but I've had no luck. When opened up the separate components, tape deck. CD player, etc that can be swapped out. If you could identify the lighting module you could likely replace it. There was a vendor on Ebay that sold what they called "refurbished" units but no idea if they are still around.

 

[Valiantpatrol]

Based on what I see, your test method is fine. Convinced your display is burned out. In a vehicle you can postulate a bad ground but on the bench any ground is pretty much direct and easily testable.