You’re really jumping around there. Maybe just check if you have Power and Ground at the dash cluster connector terminals that light up the instrument lights.

Grab a test light. Unplug the connector to the cluster.

Turn the lights on and turn the dash light dimmer clockwise to “bright”.

That C10 had 2 options for the dash cluster. One was With Gauges and one was Without Gauges (w/o gauges had warning lights instead of the gauges).

In both options, the Grey Wire on the cluster connector provided power to the Instrument lights (from the dimmer switch thru the 5 amp INST PNL fuse) and Ground was provided through one of the Black Wires on the connector. 

So attach the test light’s alligator clip to a good Ground and touch the probe to the Grey Wire’s terminal on the connector with the lights on and the dimmer on “bright”. The test light should light up if power is present.

Now attach the test light’s alligator clip to positive battery power and touch the probe to the Black Ground wire’s terminal on the connector. The test light should light up if Ground is present.

Here’s some helpful images I found. The 1st is the Connector Terminal Numbers for the dash cluster Without Gages.

The Grey Wire at Terminal 2 is the Power to the dash lights from the dimmer switch. The Black Wire at Terminal 9 is the Ground for the Dash Lights.

This 2nd image is the Connector Terminal Numbers for the dash cluster With Gauges.

Power to the dash lights still comes in thru the Grey Wire at Connector Terminal 2. But the Ground for the dash lights is on the other side of the connector at the black wire at Terminal 10.

Those images are for an older Chevy but that connector didn't change much and you can look at the Instrument cluster pinout chart to confirm that.

While you have that connector disconnected you can test your gauge illumination lamps and pcb by applying Power to the pin on the cluster that corresponds with connector Pin 2 and Ground to the pin on the cluster that corresponds to connector pin (either Pin 9 OR Pin 10) depending on your cluster type.

The lights should light up if there’s no issue within the cluster.

If you aren’t getting Power on the Terminal 2 (Grey Wire) on the cluster’s connector, then you can use a multimeter to check for power and continuity in the circuit to nail down where the problem is.

Power from the Dash Lights dimmer terminal on the light switch goes through a Dk Green wire to one side of the 5 amp INST PNL fuse.

It exits that fuse through a Grey wire that splices to the Heat/AC Control illumination, the Convenience Center, and goes to the Terminal 2 Pin on your cluster connector.

(you can enlarge the images)

It sounds like the wire from the cluster dimmer switch to the cab light is OK but not to the instrument cluster.  It may have come off a solder joint at the rheostat (cluster dimmer switch) or the dimmer switch itself is bad except when you turn it all the way to turn on the cab lamp.  When you say "head lamp switch" are you talking about the headlight switch on the stalk or the dimmer switch on the panel?  A new dimmer is available for O'Reilly, #2DS5 for $20.

Start with @Jack62 's suggestions with checking that everything is receiving the right voltage. I've had similar quirky things happen with my '79 Catalina since resurrecting it in April. The good news is that there is no BCM in these relics, and there are very few relays. Switches generally connect directly to light wiring, etc. Use a multimeter and check your voltages at the harnesses and check them in your fuse box. I use a Harbor Freight model and it works fine. That will help tell you where a wiring problem is, if one exists. My Catalina's headlights went out from a faulty dimmer switch and I checked the power at the harness before putting a new one in. The voltage was correct. Lights work with the new switch. My horn also did not work. I had power at the fuse, but not inside the pad of the steering wheel. That was a combination of a bad relay, blown fuse, and corroded leads to the horn. You may have a bad spot in your light dimmer switch's rheostat. My Catalina's dash lights go out if you twist the dimmer through a small area of its travel. 

I still bet my money on a bad dimmer switch.  At least it lasted 36 years.