2 things I'd try... take the bulb and attach 2 wires to its pins. Then connect it directly to the battery and see if it still lights the same, or at full brightness.
If it does light up fully, then take a voltage reading from behind the foglight plug. You can do this by taking push pins or sewing pins and pushing them into the wires, then connecting your voltmeter to the pins. If your voltage remains at 5v, you know there's a problem somewhere in the wiring. If it is around 12v, then you know your plug for the bulb is bad.
2004 Ford Ranger Edge manual 4.0 4x4 with 3" Body lift, 33x12.5 BFG A/T KO's, on Eagle Alloy 057 rims, and alot of mods
2011 Ford Mustang GT 5.0 manual with no mods yet
2005 Ford F250 automatic 5.4 4x4 with no mods