Good morning, Gentlemen
(nearly) EVERYTHING WORKS!!
Brake lights working, turn signal front/back and left/right is also working!
Problem was the turn signal switch. My solution was to buy a Chevy T/S switch (64-66) and change the T/S switch housing also in a Chevy C10 housing. It fits on the steering column without a problem. Colors of the wires from the Chevy T/S switch are also like the original one from GMC. The only drop of bitterness is the quality of the Chevy T/S switch housing. GMC housing material is as twice as thick > heavy duty
But with the result I can live very good
I'll start a new thread (in the next 2 or 3 weeks) with pictures showing the rebuild of the T/S switch and housing using Chevy parts. Maybe it's interesting for some other people with this problem.
The flasher is also working! Ken you are right with your test. It doesn't matter if the X is connected to the brown or violet wire. Also the L.
Many thanks to everyone for helping. Especially Ken
Forgot: last electrical problem to solve is the instrument cluster. On the round connector the voltage is ok on every single pin but on the cluster I have no signal (turn signal, ignition, etc.). Maybe the bulbs are the problem. I will test it the next days.