if you have a audio sysytem in your car that hum would be your ground on the same side of your hot going back to a amp

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THERE COULD BE A COUPLE OF PLACES FIRST CONTROL PANEL INSIDE CAR SECOND IT WILL BE IN FUSE BOX UNDER HOOD, BUT I HAVE QUESTION FOR YOU WHY DO YOU THINK ITS THE COMPRESSOR YOU CAN CHECK IT BY JUMPING OUT THE LOW SIDE IT MAY BE THAT THERE ISNT ENOUGH FREON TO START IT UP ON YOUR LINES LOOK FOR DARK OIL SPOTS ON THE LOW OR HIGH SIDE AND MAKE SURE THAT YOU THE CAPS ON THE LINES HAVE ORINGS IN THEM IF NOT YOUR WASTING YOUR TIME ADDING FREON BECAUSE IT WILL SLOWLY LEAK  OUT  ALSO , IF JUMPING IT OUT STILL DOESNT MAKE IT RUN YOU CAN RUN A HOT WIRE FROM BATT. TO SEE IF ITS LOCKED UP ONLY FOR A FEW SECONDS THOUGH HOPE THIS HELPS GOD BLESS J.B. SERVICE TECH