turn on the headlights or the heater fan

wiggle the fusable link between the + post on the battery and the bulkhead connector, and check it for volts thru it with a volt meter.

then go to the bulkhead connector and wiggle the heavy RED wire while watching to see if the lights come on. If not then wiggle the heavy BLACK wire.

If not, then go under the dash and wiggle those 2 wires again.

One might be corroded and not making contact. Then fix that which is bad, maybe by doing the MAD bulkhead connection fix.

Also check the amp gauge to be sure it isn't burned out, including the circuit board around it.