One Fog Lamp Out

Hi folks. Looking for a little expert advise. Got a 2001 Ram quad cab. One fog light went out. Replaced the bulb. Turns out its not the bulb. This is passenger side. Where would you start? Could a bad
relay or bad switch affect only one bulb? Or do you suspect a bad ground to the one bulb?
Well, I'm headed out to the garage with my voltmeter. I appreciate any guidance.

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