OK, lets start. The flasher section was wired according to the original diagram.
Violet > horizontal, brown > vertical. Yesterday I added two wires on the fuse panel to connect the flasher in the right order (X > brown, L > violet).
Without these two wires the only way to connect the flasher is with X > violet and L > brown. Please see the photos. (Small note: I think it is a aftermarked flasher. Yesterday I saw in an online shop that there are polarity adapter for turn signal relays)
Further note: when I test the voltage on the brown wire/terminal of the fuse box, where the X of the flasher should go in, I have 12V on my multimeter with ignition on.
I would suggest we do as you say. I get the truck ready, route all the wiring, etc. and then we continue with troubleshooting. In the meantime, I'll leave the T/S switch disassembled. But I think it will be in the area of April because I still have to paint the back fenders and mount the whole bed. Is that OK for you?
One question: how do you feel about the T/S switch? Can this be defective?
@Pepsi: I think it's worth a try