Sorry to revive this thread, but I realized I should clarify my response on here. I've found, that many aftermarket LED lamps light on explorers when installed in reverse. I'm not sure if it is because the way the lamps are wired is to make them compatible with C/K and standard sockets, or if it's the way the Explorers are wired and that there is a low current voltage at the ground coming from the third brake light, and the bright side that is getting voltage from the brake would then be able to ground through the marker lighting. Honestly, I never thought to check. On first Gen explorers, you can get the effect because of the voltage supplied to the third brake for cruise control, on second gen and later Explorers it seems to work differently. It may also be related to things like trailer wiring or aftermarket lighted hitch type things. Anyway, weirdness abounds with Ford wiring. That's half the fun.
But, FWIW, I had exactly the same issue on my Second Gen when I had installed one of my bulbs backwards.