The speedometer error is likely a gauge stepper motor issue, very common. Has the cluster ever been rebuilt, meaning stepper motor replacement?
There are several circuits that go to the cluster, but the main power fuse is in the underhood electrical center:
Check that circuit/fuse to make sure it's good.
Also, there is a problematic ground on the driver's side body mount, just behind the left front tire (next to the frame under the driver's door hinge area). There are two bolts and 2 or 3 wires grounded there; remove the wires and clean those grounds. One of those grounds (Ground Zone 18) is the IPC ground #2. There is also a ground up under the dash cap on the passenger side, less problematic. See the attached grounds diagram from the GM Upfitters Guide.
Lastly, you can pull the cluster (easy, many YouTubes on how) and check the powers and grounds per the attached pinout graphic. B11 is the main B+, but also verify the grounds.
Let us know what your troubleshooting shows.
Lastly,