just a few install questions

If it's a cable like an RCA we just gather it up and zip tie it, or if the extra needs to go under the carpet or something, we spread it out so that there arent' a lot of bumps from the cable.  Every vehicle is different though, and within that everyone has different equipment, so it's kind of hard to say specifically how to do it.

For speaker wires, and power and ground, we almost always cut it to length just to make for a cleaner install.  Ground cables should always be as short as possible and if you find yourself having to coil up your grounding cable, that means it's way too long and should be shortened.

If you hook up the switched wire of the dvd unit to a constant wire with a relay and a switch inline, then if you're watching a movie with the car off, when you finally decide to turn the car on the dvd player won't shut off and reset.  If you do it this way you just have to remember to turn it off whenever you leave the vehicle cuz otherwise it will stay on and run down your battery.

I just bunch up the excess wires under the rear seat.  It has always worked well for me.