To wire up the volt / amp meter, you need to put
"+" to your power source (dash lights, or ignition commonly)
"-" to ground (negative battery, or chassis) and the the
middle pin near the "CE" goes to the ground of the item to measure (fridge, light etc)
Keep in mind the negative of the item that you're measuring the current of cannot go to the battery, and must go to the centre pin of the meter.