System install cont.?

first off thank u to N2Audio for such a quick response. im trying to install a 300 watt dual amp with a 400 watt max offbrand 12' sub into a 1998 ford taurus which has a factory amp in the trunk that everything factory is currently hooked to. i'd like to keep the 6x9 that are there as their not to bad but really want this system in i've only ever installed 1 other system b4 which was really easy based on it was all bought as a package so i really need some help with this and dont want to pay hundreds of dollars for something i could do myself with jus a couple tips and a lil help thanks in advance to any1 willing to try helping me