Workhorse more than likely used GM clusters. First thing to try is to remove the cluster including the connector and then plug it back in without reinstalling the cluster. That fixes some of them. Next step is to watch youtube videos on the subject, and decide if you are handy enough to follow the instructions. Last resort is to take the cluster to a GM dealer and order a replacement. If you are unable to provide the odometer reading your cluster will have to be sent in for repair/replacement. If you can provide the odometer reading it is a next day exchange usually, you will have to surrender your old one as a core.
Once you have the "new" one it usually is a plug and play. The usually means everything should work except the steering wheel controls if you have them for the radio, etc. If those don't work the dealer will have to program the function.
Please note, the above applies only to GM clusters of that era. It is a very common issue.