Yes you have to run new speaker wires from wherever the amps placed and the new speakers.

but do you need to run new speaker wire from the back speakers if your amp is under the driver seat?? Will they (the wires) be good enough??

if you use big enough... yes

well if you are putting in new speakers back there, you might be best running new wire. And if you arent going to be using the monsoon amp you need to hook up the speakers somehow to the amp under the seat(easiest way is running new wires IMO)