1990 Buick Riviera Dash lights

riviera
speedometer
buick
lights

#1

The lights in the dashboard would go off for a while (10 minutes or longer) and then come back on for a while until recently the dash got dim and the lights went out. I had a replacement dash (speedometer) installed, but it it shows “error” in the lower left hand corner (and the dash will only light up while depressing the test button). What can I do?