1984-1989 Cluster Bad Ground
The factory grounds to the Digital Dash Cluster are a known problem. They can result in an intermittent or completely dark dash.
1) The ground wire is broken.
2) The ground wire has a bad connection to the frame (under the drivers side cylinder head, near the firewall).
3) There's a bad connection at the firewall connector.
Click Here for a diagram of the wires leading to the digital cluster.
The easiest fix for all three of these problems is to tap into the ground wires and connect them to another electrical ground. This can be connected to another ground wire like the one for lighting or the radio, or to a seat bolt, etc.
1) Run a wire from the D1/Black wire to a good electrical ground.
2) Run another wire from D3/Black with white stripe to a good electrical ground.
3) Run another wire from B1 / Black to a good electrical ground.