Dash lights

I drive a 1993 lexus 400ls. When the weather is cool the dash lights do not illuminate. But after driving 20 miles or parking the car in the sun they light up and work fine. My trip to a local garage in Arizona produces no results. I think there is a cold solder joint on the instrument PC board. What’s your guess and how to fix it. When the lights are of I never know how fast I’m going or how much fuel is in the tank.