Greetings,
You need to get a multi function meter, disconnect the battery. Make sure all accessories are off. Set the meter on milli amps and connect the meter between the positive cable and the battery positive post. There should be no more than 40 milli amps drawing. Most meters have a 20 amp fuse to protect the circuits if you forget and put something on or open a door. Start pulling fuses and when the draw drops you found the circuit. I would start at the cigarette lighter or dome light fuses, they are the most common. With no meter you could do the same but risk a dead battery if you miss.
good luck
ASE Master Technician & T2 diesel
28 years experience foreign and domestic