Be prepared to change it out in short order, there is a reason they moved to an all-metal design. The original design didn’t hold up well to radiated heat from the exhaust manifold.
From post #11, that sounds like a direct alternator to battery parallel run, commonly referred to as the "shunt wire" bypass of late. For a all stock intact ammeter-based system, it is very dangerous as it circumvents/defeats the stock circuit protection for the stock wiring if the original...
Typically the turn signal housing depends on grounding through its mounting screws into the valance. Wiring diagram indicates an added ground wire (L9D-16BK), not seeing a third wire in your pics. Should be able to touch housing to the body somewhere to test.
I see, the turn signal housing needs to be grounded to work correctly. Also make sure the forward lighting harness ground is connected at the core support near the battery.
Some cautionary notes on the later replacement switches with the extended battery feed and accessory leads. May want to consider replacing the Molex connections altogether, replace with something that can handle the current.
Ignition 1 in the engine harness, dark blue wire. Can be found at the ballast resistor, voltage regulator, one of the alternator field wires, idle solenoid and/or distributor solenoid on some models.
For the headlights, the only connection between the foot switch and the lamp sockets is the bulkhead connector. Start at the lamps, with the headlight switch on, check for voltage on the red wire (high beam) and the violet wire (low beam). Then check both sides of the bulkhead connector on those...
The Mad electrical article about late seventies trucks again? Under normal conditions, all stock, correctly loaded, there little to no current flowing through the ammeter while the engine is running, full electrical load from the alternator does not flow through the ammeter while in operation...