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Side marker lights for what? Some how that bulb socket is touching the mounting screw/screws. Maybe the same way a dome light gets its ground with a metal strip.
If the marker or clearance light only has one wire, then this wire is for power and the light grounds though the mounting hardware. If the new light has only one wire and is installed in a rubber grommet, then you will need to run a ground wire off of the mounting hardware and to the truck frame/body.
Bet if you check when installed you will get grounding through the side of the actual bulb holder because the manufacturer has a metal clip in the injection molded plastic from the socket to a screw.