Classic/Antique Car Repair: BATT. DRAIN, rule of thumb, test light

Question
ANOTHER PROBLEM SAME TRUCK, WHAT COULD BE CAUSING THE BATT. TO DRAIN DURING THE NIGHT,

Answer
A general rule of thumb to see if you have a battery drain enough to run a battery down over night is to turn everything electrical off and remove the ground strap from the battery and connect a 12v test light between the cable and the post. (If you have a radio that requires a code if disconnected be sure you have the code before you disconnect the battery) If you have a clock you may see the light burn dimly. If the light is bright, then you have a drain and you may need a wiring diagram to start disconnecting things to see which is running the battery down. First to check is the large wire on the back of the alternator.
Howard