[72lb4x4]

Sorry if my earlier post was a bit blunt...

My statement in quotes below is backwards/inaccurate. If the light goes out (current flow stops) the problem is likely to be in the cab or wiring going to the rear of the truck.

"If your light goes out with the firewall connector disconnected, the problem is under the hood. If not, try disconnecting the read and black wires from the junction on the fender one at a time."

You can also disconnect the harness connector at the outer firewall that goes toward the passwnfer side. Then reconnect the connector that runs along the driver's inner fender. Check current flow from the battery. If its zero or close, check the disconnected part that runs toward the alternator, powers the solenoid for cranking, etc. Current still flowing suggests a drain in the cab.

Disconnecting big chunks of the system as described is a good way to get closer to the problem without a lot of work/suffering.

If the truck cranks, and the accessories work with the key on, I'd guess the ignition switch and associated wiring is ok.

In a negative ground system such as this, voltage measurements are made from some point (such as the + side of the coil), to ground. Ground should be but isn't always (due to bad grounds), the frame, body, or almost any metal part.

Voltage is not measured in series as you describe you are doing.

You might try disconnecting the small plug connector at the alternator, testing current flow (Amps), then if no change, disconnect the large red wire to the alternator, and testing current again.

A one Volt drop per hour on a known good battery is a lot! If that much current is flowing, your test light would fry. In a weel ventilated area, with all accessories off, the key off, the door closed, etc., if you remove the negatie battery cable and touch it to the negative battery post, do you get a good bunch of sparks? No suggests:

Maybe a bad battery? Have it tested...

If you have a timing light, try checking for spark with it. If it lights, you have spark. Now you need "gas." Open the throttle and shoot some carb cleaner into the carb. Try to start. If it fires and dies, it needs gas.

Sorry, that was long winded... Good luck!