Get a test light, disconnect the negative cable from the battery, hook the ground from the test light to the cable, secure the probe of the test light to the negative post of the battery. If the light is lit, you have something on. Look for an accessory that may be on. If all the switches are off, pull your fuses one by one, until the light goes out. Then refer to the legend for the fuse panel to locate the circuit or device it powers. Keep in mind, it being a Ford chassis, you will have a continuous slight draw for the ECM memory and the radio clock/preset memory.