Parking lights not coming on?

The parking lights, for my brother in laws 1990 Ford F450 Super Duty diesel, will not illuminate. The headlights come on just fine, but the parking lights won't. I've checked the relay and the 15 amp fuse and both have checked out fine. They stopped working about a month ago, I've traced wires and I can't find any short to ground, I'm running out of ideas. Any help would be appreciated.

Check the blinker fluid, it might be low.

Did you check the bulbs?