Dash/Instrument Illumination: Step by Step Search

You are overthinking this ............there are only basically 3 types of lamps hookups in the clusters
1...Dash illumination. These are the lamps that light up any instrument such as speedo, the shift quadrant, the radio dial (factory radio) and so on. They all have ONE voltage source, and here is how that works
.The headlight switch has TWO power sources.

A.... One is unfused, comes from the ammeter circuit, and goes direct to a breaker in/ on the light switch and powers ONLY the headlights, nothing else. That circuit is not part of this problem

B.... the other light switch power source comes from the tail light fuse/ circuit and powers the parking, tail, and instrument lamps. That power goes through the "park/ tail" section of the switch, into and out of the dash dimmer control, and then comes out on a TAN wire and goes TO the fuse panel where that now dimmer controlled power goes TO the INST fuse. FROM that fuse, all dimmer controlled lighting is distributed.

IF YOU HAVE an illumination lamp in the cluster that does not light, then the problem is right there at the cluster. Either one of the harness connector pin/ terminals, or a loos pin on the circuit board(s) (some clusters use two boards) or a dirty copper area on the board where the lamp socket contacts. Weak/ corroded socket, or bad lamp or corroded lamp terminals.

2...OTHER TYPES OF lamps. These are fed separately. The headlight "bright", the turn indicators are all grounded, just like the illumination lamps These are all fed independently from each circuit in question.

3...WARNING LAMPS. A warning lamp such as OIL pressure IS NOT GROUNDED. This type of lamp is fed full battery switched power from the "key" and then the low or ground leg goes to the sender such as oil sender. This sender provides a ground when the oil pressure is low