1986 Cadillac Brougham

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.
July 11, 2010.

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