speedometer works but display is hardly visible
Assuming the dash is at full illumination and it is still dim then the Halogen lamps are very weak or one has burned out, as these lamps only provide light for the speedometer, you will need to remove the screws holding the insturment panel and bezel out then move the cluster out just enough to gain access to the top of the cluster too replace those lamps, they are made just for this application so make sure that you ask for the right lamps and don't touch the bulbs when installing them as that will shorten thier lifespan. I haven't seen an LED replacement yet for these lamps.
Yup, one of the bulbs is out.
Looking for a Used Thunderbird in your area?
CarGurus has 1,005 nationwide Thunderbird listings starting at $5,999.
Search Ford Thunderbird Questions
Ford Thunderbird Experts
Related Models For Sale
Used Cars for Sale