Instead of using a light bulb or meter, I like to use a buzzer. You can get one at Auto Zone. I think they have them that you can plug into your rear lights for a backup buzzer.
Anyway, disonect the battery and place the buzzer between the batt and cable. If there is a draw the buzzer will sound. Now you can crawl all over the truck disconnecting and pulling fuses without haveing to look at the light or meter every time.
When you find the right one the buzzer will stop sounding.
Eric