If I understand how you are hooking up the meter, you are not measuring the draw on anything when you disconnect the ground cable from the vehicle. When you disconnect the ground cable from the vehicle and measure voltage between the cable end and the battery positive you are just measuring static battery voltage. Once you take the ground cable off the car, it takes the rest of the car out of the measurement.
You are trying to use a voltmeter to measure current draw and it will not work that way. A voltmeter can only measure the voltage difference between two points. If you want to measure current draw you need to disconnect the ground cable from the battery and put an ammeter between the ground cable end and the battery negative.