No mate one white is live the other will be a earth from the control unit, you would need a wiring diag to see which is which, or you could put your black muti plug lead onto battery neg and probe into the car side off loom! The earth should read zero and the live should read 12v (or maybe 4v in your case) It don't matter which way meter is if it's wrong you will get same reading but with a - in front of the number! If you have a perfect circuit yes you should see 0 volts as would be no voltage lost