You can bench test the 84-86 Daytona/Laser digital dash quite easily.
You will need a 12 Volt DC source, car battery can be used but is risky, if you cross wires or connect to the wrong pins a car battery has more than enough current to destroy electronics and melt wired together.
I prefer two 6V square flashlight batteries connected in series to create 12 Volts, or if available to you, an electronics 12V bench power supply.
You will need insulated alligator clips to connect to the pins in the connectors at the back of the cluster to the power source, or if you have the pigtail/connectors you can simply twist the wires together and connect them to the power source.
You will only be connecting to pins on the 21-pin black connector (Note: I have seen the 21-pin connector as other colors too!)
Pin 3 and 10 and 21 will be connected to positive 12 Volts DC
Pin 6 will be going to negative 12 Volts DC
That is it! All of the displays should come on and the trip buttons on the left side should illuminate. All of the inputs (speed sensor, tachometer, idiot lights, etc.) are indepentant of the digital cluster so there should be no concern or problems with them beside idiot light bulbs being blown (which can be checked easily by disassembling the dash and pulling the bulb and see if it is physically blown... Replacement Bulbs are #37 Mini Bulbs.
The two bigger bulbs you see lit up in the picture above are for night illumination, they are size 194 bulbs (same as bulbs in exterior marker lights) and easily replaced by pulling off the green lens cover and pulling the bulb straight out.
Hopefully everything is fine, however if one or more of the displays is dim or is lit up unevenly it means that the VFD (Vacuum Flourescent Display) displays is on the way out, unfortunately they cannot be repaired (***see comment below) and do have a life span. Sadly eventually everybodies digital dashes will fail over the next couple decades.
If you have the one bottom green LED lit up for the tach, that means it is ok. If no LED is lit up during bench testing that means the LED array circuitry has failed, that required extensive knowledge in electronic troubleshooting to repair.
If the digital dash VFD displays do not light up and everything is connected correctly and battery voltage is sufficiant, it is beyond basic repair and will have to troubleshooted at a electronics level.
If the three switches near the center of the board (for trip, KM/Miles, reset) are not working they likely need to be replaced, part no. 612-TL59AF160Q from mouser.com are the replacement buttons, and are simply replaced by desoldering-removing the old buttons, and soldering in the new ones.
***I must note you can, in theory, adjust/increase the on board 30V power supply in the digital dash to brighten up dim VFD displays. I have never had to explore this fortunately, though the displays will slowly still degrade over time. I am not sure if there is a potentiometer that can be adjusted with a screw driver somewhere in the 30V power supply circuitry, or a resistor will have to be located, calculated and replaced to up the voltage and restore brightness to the displays.