Audi A3 8L 2001 1.9 TDI: Rear Fog Light not Working Despite Bulb and Switch Replacement

Hello
In my A3 8l 1.9 Tdi 2001 the rear fog light is off. I replaced the bulb and it didn't help. I checked the 36 15A fuse and it was OK and the light was still OK. I started to trace the gray-white cable from the light switch. It turned out that after turning the switch to this light there was no voltage. So I replaced a broken switch. I connected and ... Voltage appeared after turning the switch but the bulb as it did not light, it does not light. Correctly the light should be lit in the left lamp on the driver's side. So I started looking for a gray-white cable in the cube entering the lamp but I did not find it. It was next to the second lamp, which surprised me because the car is from Europe and the beam looked original. Well, but I moved the light bulb to the second lamp but it was not there either. I'm in the spot now. I will add that on both lamps the voltage in the cube is only on one pin in the left lamp on the gray-yellow cable and on the right on the above-mentioned gray-white is the voltage. Somewhere I read that the original hooks in a3 or rather the sockets have some microswitches to turn off the fog after connecting the trailer to the car. So I looked into this 7-pin socket (maybe the original problem here had 13 pin probably) I measured the voltage on three male pins and there was power supply on two 12v and on one very small. And worst of all, I didn't notice this microstatic there. Somewhere I learned that the voltage to the rear fog lamp does not flow through the relay so I rejected the possibility of its damage. I WOULD EXTREMELY ASK FOR HELP In advance Thanks

I put in the fog scheme




If there is voltage on the gray and white of the switch (the light on the board is on) then everything is OK.
You wrote that in one lamp you also find gray-white and there the voltage also appears when turned on?
What about the socket itself?

As for the socket of the trailer, there is a bridge there, not a micro switch

Unless you have a fog switching relay (car / trailer) but then there will be no voltage on the lamp cube.

It is in this problem that the switch is not yet set to fog and on this gray-white cable in the lamp cube is voltage and the switch does not come out voltage

I suspect you have a hook module plugged into the lamp connectors and therefore the color of the wire does not match on the left lamp. Look for the black box around the left lamp.