RCA cables from HU to trunk...

I would think 10ft would be plenty. Down the passenger side is the easiest route I could find. I just upgraded my entire sound system, including routing a power cable from the boot battery to under the passenger seat for a low profile sealed sub. Most of the door sill panels pop right off with a little tug. it's great to have a car that's assembled like a stack of legos

I also installed a backup camera under the rear wing, just behind the brakelight, and needed about 12ft of RCA for the video feed line. However I had to route that through all the internal hatch door panels and through the ceiling following the rear wiper fluid line (what a b****), so I think 10ft should be enough for your application.

 

I don't think 10ft. would be enough for most applications. The kit I used to install my subwoofer had a 20 ft. cable, and IIRC I had ~5 ft. of extra I had to tuck away. I ran it from the back of an aftermarket headunit in the center console to the driver's side vent by the door, then down to the cable "tray" running along the left side of the car under the doors, then around the back seat into the trunk. Granted, my amp is near the back of the trunk on the driver's side. I wouldn't get less than 15ft, especially as there's plenty of space in the dash to tuck away extra wire with a couple of cable ties for a clean install. I think it will be much easier to run the RCAs on either the left or right side of the car, then to take out the carpet and run it down the middle. If you have a tape measure it shouldn't be too hard to measure distances and estimate (just add an extra foot or two to your estimation to be safe).