Interior problem
1986 Cadillac Brougham V8 Two Wheel Drive Automatic 67235 miles

hi, ive had a constant drain in the battery. so i got a tester to see what has been draining the battery so badly, it turned out to be a spark plug and i have no idea what it does. it is labeled as body, 20 on the spark plug. my car came with no owners manual so i cant look it up and would be appreciated to know what it does and if its a necessity to keep it plugged in because it is constantly drawing power even when the car is off.