Car Audio System in a 90 LX v8 stang

[ziggy_2k]

I have a 1990 LX V8.
First off Here is my dilema I went to install a deck and used a common wire harness kit that I purchased from tweeters. Only when I pluged it in there was no sound. I tested the deck on a speaker that I had our of the car and it worked. I looked at the wires in the stang and realized that only 5 wires running in to the stang 4 positves and 1 negative I assume. So the wire harness i bought isn't working. Any ideas besides running new speaker wire? Perhaps I have an amp from the factory? If so where is located? I got the car a few months ago and really haven't had a chance to rip it apart and put it back together. This is my 3rd stang all the others were easy to run a harness into.
Second off I plan on putting a sub in my convertibles trunk. Yes it sounds silly i know. But I have done it in the past and it doesn't sound half bad for the HUGE sacrafice of trunk space. I was just wondering if anyone has any suggetions. I use a truck box and put it on the left side of my trunk.
Lastly if i need to rewire all my speakers has anyone run speakers wires in a convertible? If so any suggetions? Doesn't sound to inticing but the things i do for sound. I already attempted to pull the speakers from the door unsucesfully. I unscrewed every screw but i couldn't pull them. Any ideas?
Oh just a few more questions. How hard is it to get to the speakers in the dash(are they some how bridged to the wires going to the door)? How does one get the back seat out(i forgot how)? Does anyone have any suggetions that might help please let me know they will be greatly appreciated?