[Nonfractal]

Jorgen,
Looking at the pic of your broken guide wiring , it looks like  the end of the wires were soldered. 
Soldering makes it easier to insert the wires into to the guide but makes the wire brittle and prone to breakage like this, especially when using set screws (grub screws) to fix the wire. 

Everyone has their own favourite motor wire and you will find lots of different opinions on this.
My own "go-to" wire is pendle slot's own  braided silicone. 
An ideal motor wire :

- is thick enough to carry the current unimpeded.
- is stiff enough to gently centre the guide
- is soft enough not to break in use.
- is skinny enough  not to rub on the car interior.

Different bodies and motor layouts (AW/SW/IL)  present different challenges in wiring layout .
You just have to experiment until you find a method that works for you.
You might have to lift the front of the body up slightly using a washer on the body post to get the clearance needed.

Hope this helps.
Alan