Great support
First I will test the voltage directly on the panel fuse if there is voltage for the flasher but I'm with you. Main problem was that the turn signal harness was not connected. I will work through your points and questions. Would the turn signal lever work without the tail light for a first test? The rear lights are not connected at the moment.
I'm a total beginner but slowly learning a bit more about the car electronics and thank you for the advice with the ice pick volt testing light.
Further question: on my truck the pre owner used the following sockets for the turn signals. Think they are for the taillights and they don't really fit in the opening of the metal housing. Are there "original" ones available?