1997 Ford Explorer, I checked the filaments and all is fine, when I turn on the "parking lights only" the head lights come on but the running lights out back won't glow, brake lights work fine

When I turn on the parking lights, the head lights shine but the running lights/tail lights
don't burn out back, I checked the filaments and they look good and even took them out
and swapped them out, it is not a fuse or else none would work, what else could it be?
The brake lights work, even the 3rd light works fine.??