You can cut and splice but I would not, what i usually do is just add wire to the wire I am tapping this way I can replace it back to 100% stock and not worry about it latter on or wonder what I did with the other half, just make sure you sodder and tape the wire good. Ford does give you enough space to do this or at least they did in my Dads explorer and in my X and Explorer, I will try to take the weekend to test the wire and post pictures and a diagram. Plus you can also use those wire crimp tees those work to.

But yes as long as the Factory DVD entertainment system is powered up and you have the leads coming for the audio you can convert them to the RCA Line Level Convertors plus you can also dial in the output voltage to the RCAs so the DVD wont be so quite when you play a movie, and if you opt for a FM modulator, this way if you have a Nav system installed you dont have to worry about anything but tuning into the preset FM Station.

I will post more info for all on the Explorer and the Excurion the pinciple is the same for all vehicles that employee the same style of system but you will still need to take proper measures to test things first.

PS you may want to try this it is very simple and works with your Pocket PC or Laptop and has voice guided directions, I tried it and it works very well, sell in canada for $300 CDN since all you need is a Pocket PC Handheld you dont need to change your headunit or do anything other than plug it in, but you need to have a IPaq or Pocket PC that has a Compact Flash Card, I have the 2215 but the newer version 2410 or 2750 supports the CF cards and HP also has its own wireless Bluetooth version of the GPS Navigation, which would be nice to try since it operates off of Bluetooth it is wireless and not like the Ambicoms version which has one wire from the top of your deviece to the GPS Sensor. http://ambicom.com/products/gps/gps-plus.html