Parking light does not function

Hello, everybody.

My parking lights don't work.
How can I find the mistake?
What do I have to do to make the standlight functional again?

Greetings Markus

Ps: The forum entry with the standlight (does not work) is also mine.
It's a false entry.
I'm prescribing.

Sorry to all who have already read it and may know the answer.