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Came across this same plug on my '90 BII for the first time just last week, when I was installing my new hood pad, and wondered what it was - now I know!
I put in my own version of DRL: 4 of those yellow/reddish LEDs spaced evenly inside the grille, like modern trucks have. I really like the look. Have them turn on with the ignition, cost all of maybe $10.