Citroen C3 help needed

[myglaren]

But I still don't understand why the four-way flashers don't work.

Try repeatedly pressing the button for them. (Hazard button).
The button contacts are known to corrode as they are used so infrequently, lots of pushing the button cleans the corrosion and gets them working

[ozvtr]

The vehicle's central locking system was only installed later and disabled, so I'm trying to connect a universal lock mode again.

I'm sorry but can you clarify this? What do you mean by "only installed later and disabled"? The car comes stock with central locking via the key fob and the dash button. Only the key fob will cause the indicators to flash.
Does the hazard button on the dash cause the indicators to flash?
Does the central locking button on the dash lock the doors?

[ozvtr]

As for the air conditioner, everything is working properly except the compressor.

You said there is a communication fault with the A/C panel. The compressor wont work if the A/C panel is not "talking" to the BSI. You will need to sort that out. I suspect there is a CAN BUS panel installed in a VAN BUS car. Or there is a fault with the A/C control panel. Seeing as you don't know the history of the car, anything is possible.

[Halil]

The hazard warning flasher button does not turn on the signals and the lock button does not lock the doors.

[ozvtr]

Wow! This is like pulling teeth!
Do you have an after market radio installed? If so, completely disconnect it.