Says 1983 - but it doesn't really matter - all model years are layed out the same:
Where does the junction block on the firewall get its power from? The batt/ alternator or fused from the fuse block? Got a wiring diagram?
The JB uses no power, it is essentially a terminal strip. But I know what you mean.
The short answer to your question is: From the battery.
But it isn't a straight shot from the battery positive terminal to the line side of the FW JB. To get that point, there must be continuity:
1. From the battery positive cable to the "B" terminal on the starter solenoid
2. Through a fusible link. That link is located very close to the solenoid "B" terminal: