Reddit mentions of InstallerParts Cable Management and Organizer Cover - Expandable Braided Cord Sleeve Computer Cord Organizer 1/2" (12.7mm) x 100Ft (30.48m)

You can run wiring through the rubber grommets behind the steering column and near the fuse box in the glove box area. http://www.fordfiesta.org/forum/threads/378-Fiesta-ST-subwoofer-wiring-project

I personally ran my wiring underneath the Fiesta, as I was using 0 gauge wiring which is quite thick to run through the firewall without drilling. Power wire with an inline fuse from the battery goes underneath the car to the trunk. There is a nice cover on the bottom of the car and you can run it next to the brake lines. I also wrapped the wire in abrasive resistant wire sheath. The trunk has a very convenient rubber gasket that you can feed the wire through, it keeps it waterproof.

For the signal wire I spliced into the two rear speakers and then ran those wires back to the trunk. There is a channel on the edge of the doors to run the wire, just pull up the rubber. In order to get to the speaker wire you can pop off the plastic cover on the middle column. I only pulled it out a few inches and was able to splice into the wires. The splice speaker wires will be converted to RCA which will then be fed to the amp. Just make sure your amplifier has proper filters.

Power wire to battery with inline fuse.

Notice wire sheathing for protection.

Bringing the power wire into the trunk - there is already a rubber grommet that can be used.

Braided wire sleeves on Amazon (get the right size depending on what wire gauge you use).

Kick panel channels under the weather stripping and pulling out the plastic cover to expose the rear speaker wiring.

My wiring setup.

Shit diagram of the wiring setup.

Bass controller in the glove box.

Sub.

Fiesta ST speaker wire color codes:

• Fiesta ST Left Front Speaker Positive Wire (+): White
  
• Fiesta ST Left Front Speaker Negative Wire (-): White/Brown
  
• Fiesta ST Right Front Speaker Positive Wire (+): White/Violet
  
• Fiesta ST Right Front Speaker Negative Wire (-): White/Orange
  
• Fiesta ST Left Rear Speaker Positive Wire (+): White/Green
  
• Fiesta ST Left Rear Speaker Negative Wire (-): Brown/Yellow
  
• Fiesta ST Right Rear Speaker Positive Wire (+): Brown/White
  
• Fiesta ST Right Rear Speaker Negative Wire (-): Brown/Blue

Ok, I guess I'll give away my secret.

I use this braided cable cover, which gives the wire a bit more stiffness, so it doesn't tangle as easily. :)

https://www.amazon.com/InstallerParts-100Ft-Expandable-Braided-Diameter/dp/B01DEX449C/ref=sr_1_1?ie=UTF8&qid=1482972991&sr=8-1&keywords=braided+cable+covers