[DetroitDan]

if you're talking about the 5 marker lights that go across the tailgate, then I'm confused. I've only ever seen them with one bulb in each light. are you sure they aren't some aftermarket two bulb marker lights that someone put on a tailgate?
I had a couple sets of the cheap aftermarket lights that I put on a couple trucks, wasn't a worthwhile investment. The white plastic backing witha white bulb and thin red lens made it light up almost pink. I took them apart, which they arent made to do, by cutting the backs with a razor knife. Bulbs were soldered in and not replaceable, so instead of putting red bulbs in I ended up painting the inside of the lens red and gluing them back together. Looked really good, had a bright red almost led look. Looking for replacements now for mine, I have a tailgate with factory marker light housings but last guy swiped all the bulbs. I forget the part number but they are reasonably available.