There's a gray wire from the fuse panel that feeds the dash lights and the heater switch faceplate. GM made two styles of clusters for our trucks, the seven gauge, and the three gauge with the idiot lights. Assuming that you have the seven gauge the gray wire connects to pin 8 on the cluster plug. It connects to pin 12 on the idiot light plug.
The fact that your original cluster lights work tells me that there is no problem with the gray wiring or the power feed to it from the headlight switch.
I would check the other clusters outside the truck with a lead from the positive terminal on the battery to either pin 8 or pin 12 depending on which dash cluster you have. Then ground the cluster with another wire from the battery negative.
From what you've done so far it seems to me that you have two clusters that have something wrong with them. Below is a diagram of the seven gauge cluster showing the wiring circuits. The wiring key is on the left side.The gray line to the lights is power and the orange lines is ground. By connect power to the grays and ground to the orange your lights should come on if the circuit board is OK. You could use an ohmmeter as well and test for continuity.
Here's the back of the cluster, the red wire is a power takeoff for the tach and it's off the pink wire to the fuel gauge.
