In my Octavia Mk 3 Estate I have concealed the cable from my rear window dashcam everywhere except the rear window to rear passenger door pillar. I can't see a way round hiding the cable here as the plastic trim is very stiff and stands proud if you do manage to get the cable behind it.

 

Has anyone any ideas about cable routing bearing in mind that the length of cable necessary is very close to the technical specified length for a USB to mini USB cable?

If your just providing power to the camera then you can in theory go further than 5m with a good PSU. It's the data that's the problem due to voltage drop