05 nissan altima amp wiring situation

HI, new to the site and Im trying to install a new 4 gauge amp kit into my car, I wanted to run the positive wire alont the floor boards of the left side all the way to the trunk, but I tried for two hours and I couldnt seem to get through to the engine compartment I cant find any hold that lead out, also Im using a hi/lo adapter to get rca pre outs on factory stereo till i purchase the new one, I found the back rear wires to splice the device into does anyone one know which wire colors are positive and neg. my care is 2005 nissan altima 2.5 non bose system please any help with pictures or anything would be very well received.!!!!!!Update::: I got the positive amp wire installed, and I have the amp running, now Im kinda stuck with two more problems, I hear base, but it hits nothing like it used to in my other car, now I think it might be two things, Like i mentioned I knew which wires were the rear ones for the hi/lo adapter but I couldnt tell which were pos/neg so I dont have a wireing guide nor do I know where to get one, so I think I might have the wires crossed, also I dont think it makes a differance but it is worth mentioning instead of running straight rca from the hi/lo back to the amp I have a simple gain adapter in between which is just a set of rca in puts and outputs, which lets you adjust the gain on any system. I dont think thats the problem however. Also too one more thing due to not having a wire guide I have the amp hook up to constant electricity, I just picked the biggest wire on the back of my radio and figued it was power, I need to identify which is a wire that will only give power when the radio is on. really I think i could figure that out but I dont know why the base is so week. as for the type of woofers I have two 12in sony xplod 1000 watt , and its the sony xplod 1200 watt amp, like I mention before I had this system in my previous car and it screamed, now its hardly even more than an annoyance, again this is all with the stock speakers and stock face plate with a hi/lo adapter for rca preouts and a universal inline manual rca gain adjusterTherrin Hunter