The rheostadt (variable resistor) is dirty or broken in continuity.. Pull off the 3 terminal connector and jumper the 2 orange wires using a short wire lead with male tab 0.25" terminals. Now you will get the full 12V to the dash lites. The other terminal is for the Dome lite, and you should run a short lead back the switch for this.
For the Dome lite circuit to work rite the dash ground circuit must be tite and secure.
Note that for the dash lites the only question about grounding is the bulb socket itself on the circuit board. Make sure the circuit board has a good solid ground, as there is no grounding through the dimmer switch for the dash lites. Its purpose is to
reduce the 12V down to 5V and this is done on the 12V+ side which is above ground..
The single wire is for the Dome lite circuit.
Hope that helps...