Maybe start at the select switch on the dash and trace the harness back towards the firewall. That should give you a good idea where they transit the FW. They have their own 4 pin bulkhead connection block similar to the main bulkhead connector - just much smaller. A round connector plugs into the bulkhead connector on the engine side. After that they probably run rearward in the rear lighting loom. It might be easier to start looking for them there.
The wiring for this setup is kind of involved. But if you're interested, I'll try to help explain the way the system worked - and how/why it was wired the way it is.
The first thing is that all of the following will only apply to the fuel tank level circuitry used in CK 1/2/3 standard pickup trucks with the
NL2 RPO option (i.e. dual tanks mounted outside of the frame rails). Also, unlike the later model CK series, the production tank was the one mounted on the RH side - the auxiliary tank was on the LH side.
The solenoid power circuit is simple enough - either on or off - so it isn't discussed.
If the NL2 option was not requested, a single PNK lead started at the instrument panel. Then it penetrated the firewall, ran down the RH frame rail and terminated at the sender connection on the base model (production) tank.
But with the dual tanks, two inputs were required for the gas gauge. The solution was to splice a long TAN lead onto the end of the base PNK lead - and then run that TAN lead all the way forward on the LH side to the tank transfer switch. Additionally, a lead from each tank sender (LT BLU & TAN/WHT) was run up the LH side to the switch. When the operator switched tanks - by supplying power to the transfer
solenoid - another set of contacts within the switch shifted. This controlled which sender lead (LT BLU or TAN/WHT) was connected to the common TAN lead. That allowed the online tank to provide the gas gauge with the correct electrical signal. See the attached dwg and maybe it will help explain:
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