I ran into this problem years ago when I got my first Reatta. I would get "the Show" [when all the buttons would light up, the car did the self check, and the Reatta logo would come on] but wouldn't light up at dark during normal driving. At the time I wasn't very knowledgeable about these cars, but because the car lite up during "the Show" I was pretty sure it wasn't the inverters, so for me it was replacing the photocell.
If you go into the dash be sure you make the "tool" needed for removing the dash pad, or you will be back asking about where do get a new dash. The instructions are found in the "How To" section on the top of this website. Look around and you will see what the owners of Reattas and Ronnie have done for you.