I just asked my buddy and he says you can tap that speaker wire anywhere...doesn't have to be behind the dash. Also for the remote turn on wire...you can run it from a switched circuit (cigarette lighter would work if it's switched on/off with the ignition) or on the stock headunit, tap it into the power antenna wire. If your son's dash comes apart similarly to mine, it's very simple. 3 screws and a few clips you just gotta gently pop out. Also, when you run the power wire off the battery, make sure you run an inline fuse...I usually put them under the hood near the connection to the battery. Don't forget to ground the amp if the previous owner pulled all the wiring off the amp. Just run the ground from the ground terminal on the amp to some bare metal on the body/chassis. You want to keep that wire short...preferably around 1-2 feet tops. I usually drill a hole in the body near the amp, grind the paint off around it and then using a nice big screw, screw it down tightly to the metal. Also make sure you use the right gauge wire for the amp or you may end up with a nice little campfire in the Jeep.
I usually use this place for my amp wiring needs:
http://www.knukonceptz.com/