86 TBird Speedometer Dark
It could be a bulb that has gone out. There is a bulb or maybe 2 that backlight the speedo.
Bob's right, those bulbs are very fragile after they have once been turned on, they are not just the typical dash lamps, they are halogen lamps and require special care when replacing them, then instrument cluster will have to be taken loose and pulled out enough to gain access to the lamps. HTH
Search Ford Thunderbird Questions
Ford Thunderbird Experts