I have fixed two windows in my 159 SW. The first one was easier, as I bought a whole new mechanism (the guide, the "shuttle", the "winch", with the cables attached out of the box). For the second window (rear passenger's side) I opted for "repair kit", which consisted of new cables and the shuttle only - which was half the price of the whole assembled thing.
First step, put the cables with one spring end into the shuttle (it has a groove for it) - after you figure out which one goes "up" and which goes "down". Then snap the shuttle onto the metal guide. Thirdly, put one end of each cable "armor" in the shuttle's metal guide, routing the cables themselves onto the rollers. Now you must figure out which cable has to be wound and which unwound (depends on shuttle's position on guide) and, while keeping the long spring compressed towards the "winch", start winding the cables. I had to ask another person for help to keep the spring with the guide rod in place, while twisting the "winch" (in a perpendicular axis to it's normal operation) to tension the whole thing because there was absolutely no tolerance on the cables. Managed to put it together, barely, after an hour or more... Not a pleasant experience but it's been working for like two months now without issues. The procedure was so annoying that I didn't even think about taking pictures...
The original shuttles are really brittle, I had broken two so far. The whole mechanism fails because the shuttle breaks - it makes the winch rotate too far (normally the shuttle stops on the guide and prevents that) when it breaks and detaches from the glass. This in turn usually snaps the cables out of the winch wheel and they get mangled onto the motor axle (which thankfully stops the window from falling down, usually).