What would cause one fog light not to work (bulb is good)?

I would check for voltage at the light that does not come on. If there is voltage then I would suspect a bad ground somewhere near the light.

check all wiring in the fog lamp unit for corrosion around all electrical connections

Ground wire

Bad connections at the bulb. Fog lights are down low where they get water and dirt on them. Clean the connections and the socket where the bulb fits. Wiggle it around a little and you will probably see it light up.

Use a multi meter with pointed probes and slightly pierce the wires directly behind the socket which does not work. If there is 12 volts there, then replace the socket. They are usually available at various auto parts stores for your particular make and model. They come with pig-tails to make the connection. I suggest you use shrink warp tubing over the connection points and follow up with a little electrical tape if that is the problem.

I have only one fog lights coming on in the passenger side , but in the driver side I don’t have even voltage. What can be ?

O have the same problem but I have 4.7 volts on the positive terminal the ground it’s ok