This section is a restructuring of the original Service manual upload to the forum in an attempt to make it easier to download and navigate the PDF files. The originals are still there if this changed structure doesn't suit you. I should say...
www.fiatforum.com
It's in multiple PDFs but the wiring diagrams can be found in '55 - Electrical system - Wiring diagram.pdf'
I've included screenshots of the two pages for the turn signals here hopefully that can help you.
Thanks for your answer.
I already knew those diagrams, but I don't know where the flasher relay is, since it doesn't appear as such in said diagrams.
Keep investigating.
I agree, looks quite straightforward to change.
The first part number (7789496) is for B's before 15/01/1996, the second part number (7573490) is for for B's after that date
Good luck!