The hardest part will be using a relay for all the switched powers. Basicly you'll need to get a large guage wire, 12 guage should do and run it to the battery positive terminal or the constant power at the column. Thats the top terminal, terminal 87.
All of the switched powers should go to terminal 30.
The switched power or remote wire of the radio, if aftermarket, should go to left terminal, terminal 86.
Ground should go the the right terminal, terminal 85.
You can ground the units to anywhere you want but I would suggest using the same gound point for the DVD player, radio and fm modulator. Don't just tap the factory ground wire. Make a new one with a larger, 12 guage wire once all the other gorunds are connected together. Or just ground the DVD, and FM mod to one spot and make a new radio ground at that spot.
I suggested the above to aviod audio ground loops. you don't have to but it is a good idea.
Your Video DA may also have power and ground. If it does just attach the power with the rest of the switched powers at the relay and the ground where ever is easiest.
Once everything else is connected you can experiment with the parking brake. I'm not sure what it needs to trigger on the DVDplayers out puts. Its either power or ground. Just place the wire to a ground point and see if it comes on, if not tap it wo constant power and see if you have an image then. If ground is needed then just ground it, If power then attach it to the array of other switched power wires.