Power is supplied via a fuse, so that is either good or bad. The other thing could be related to a weak ground. Now, interestingly, the same ground that works for the speedo is for the entire cluster as is the power supply.
I would suggest confirming there are no issues where the connector plugs into the rear of the cluster. If that is good, I have to assume (which I know isn't a great idea) the cluster itself is bad other wise you would lose the speedo as well.
I attached a pic below showing the ground and power supply to the cluster. If you determine the cluster is bad, here are the directions for replacement.
2005 Pontiac Grand Prix V6-3.8L VIN 2
Instrument Panel Cluster (IPC) - Replacement
Vehicle Instrument Panel, Gauges and Warning Indicators Instrument Cluster / Carrier Service and Repair Removal and Replacement Instrument Panel Cluster (IPC) - Replacement
INSTRUMENT PANEL CLUSTER (IPC) - REPLACEMENT
INSTRUMENT PANEL CLUSTER (IPC) REPLACEMENT
1. Remove the instrument panel (I/P) cluster trim plate.
IMPORTANT: The ignition switch must be in the OFF position when removing the instrument cluster.
2. Remove the instrument cluster screws.
3. Pull the instrument cluster towards you and disconnect the electrical connector.
4. Remove the instrument cluster.
1. Connect the electrical connector to the instrument cluster.
2. Align the instrument cluster to the I/P.
3. Install the instrument cluster screws.
NOTE: Refer to Fastener Notice in Service Precautions.
Tighten the screws to 2 N.M (18 lb in).
4. Install the I/P cluster trim plate.
Let me know if this helps. Also, I noted you are a mechanic. If you want the entire schematic for the cluster, let me know.
Images (Click to enlarge)
Saturday, March 7th, 2020 AT 7:51 PM