You need to start with a parasitic draw test. That will test the whole truck. You would cycle various switches to catch a intermittent issue also, while watching multimeter. More finite measurement than watching a light bulb. If you see a leak/draw, even in the milliamp range, and cycling things on and off does not identify, then you begin wire disconnection one at a time while watching meter.
Install meter in series with negative battery post and cable. Set DC mA.
