As wired from the factory there are only 2 leads (or 3 if A/C equipped) that connect to the firewall junction block.
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Without A/C, only the LH (looking fwd) terminal post is used. It is the bigger of the two - maybe 5/16".
At this terminal, power is in from the solenoid "B" terminal on one lead. That lead is protected by a fusible link down at the solenoid.
Power is out - via another fusible link - to a splice. One leg after the splice is to the BATT terminal on the alternator (the big one with a stud/nut). The other leg transits the firewall at the main connector, splits and feeds two circuit groups. One leg is to the BATT 2 & 3 terminals of the ignition switch. The other is direct to the BATT bank of the fuse block. The BATT bank supplies fused power to the T/L CTSY, HORN DM & ECMB circuits. It also supplies the 6 BATT plug-in sockets above the fuses.
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If the truck is equipped with A/C, the power for the blower motor is tapped at the RH (looking fwd) terminal. Note that the stud is smaller, 1/4" IIRC.
The blower/fan wheel used with A/C ventilation systems is of a higher capacity than the blower used with heat only systems. Due to the increased current required to operate the fan, battery power is delivered to the motor via a relay (controlled by the fan speed switch) - rather than just straight through the control panel switch like the heat system.
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The above describes how the truck was originally wired at the factory. But, because the two terminals are common within the block body, which terminal is used to connect which lead can be changed to anyway you want. They are only restricted by the ring terminal size.
One hint: While you have the JB off the firewall, take a close look at the back. I don't recall the exact details, but I have had a situation once where one of the studs broke free and somehow came in contact with the firewall sheet metal. The stud was probably overtightened at some point and cracked the Bakelite casing. If nothing else, a piece of rubber inserted between the back of the JB and the FW should prevent that from happening.