When i ran my wiring for my amp the battery live i ran down the chassis and into the boot via the rubber grommits that were for the tow electrics
The phono jack leads and radio live i ran down the dash, under the kick plates at the bottoms of the doors (you have to lift them out and feed the wire under the carpet) and then under the brear seat carpet to the boot.
You can piggy back speaker wiring, just make sure your using suitable cable
I've since turned the boot live into a fuse box and run the amp, power outlet (for fridge) and 2nd battery controlled caravan electrics from one handy location
:D