brake power booster

I recently installed a rebuilt brake power booster on a 1986 diesel Isuzu p/u. Located at between 10 and 11 o'clock on the front of the booster is
a small, male threaded component, with a single wire coming off it. The wire is live when the key is turned on. Don't know what this component is and haven't been able to get any help from Carquest, where we bought the booster. The old component is threaded slightly smaller than the hole on the rebuilt booster, but before I can get a new one, I need to be able to tell someone what it is I'm looking for. Any ideas?
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