Well, I didn't expect it to be easy but I expected it to be less time-consuming. :)

Here's the solution for the Arosa:

There's one or two cable trunks that pass the firewall, but they're sealed so well that it is impossible to put another cable through there. Apart from that, there's only the mechanical stuff - with one exception, a hole near right under the pedals behind a plastic cover. However, this is too close to the engine and the pedal mechanism, it's dangerous to put the cable there.

Standing in front of the car, the solution is a hidden hole at the bottom right corner of the window. There's a cable that is used for opening the motor room which runs "stealthed" at the inside of the motor room to the lock. One will have to remove the rubber water seal and carefully bend away the plastic cover that is attached to the window, then you can reach the cable break-through with your hand.

The other side of the breakthrough can be found when removing the cover of the fuse box at the inside (you will have to remove the fuse box and the surrounding part of the armature). Grab through the hole and then go straight up (I wasn't able to find the hole itself, but I was able to find the cable).

Make sure that the cable, while pushing through the hole into car, doesn't get behind the isolation at the firewall, eventually you will have to remove it a bit.

Anyway, after that - it was not too hard. :)

Till