2002 dodge caravan sub

Here is the simplest way to do it.

Get one of these, or go to any car audio store and ask them for a Line Output Converter. This will use the factory radios speaker wires and convert them to RCA cables.

http://www.hifisoundconnection.com/Shop/Control/Product/fp/vpid/1583043/vpcsid/0/SFV/30046

I am giving you the wire tap version as it will be quicker, easier, and allow you to take everything out. Personally I would solder everything so if you like you could replace the term "wire tap" with solder if you like.

Pull the factory deck and use wire taps to connect the converter and run the RCA under the carpet to where you want to mount the amp. With the radio out use a test light or DMM and check the blue/pink wire going into the radio, this wire should be the 12v switch for the radio which makes the radio go on and off when you turn the keys. Probe the wire and make sure that with key off there is no power and with key on there is power. As long as that checks out use a wire tap and attach a remote wire there. Run the remote wire to the same place you just ran the RCA's. Considering you will be taking the seat out to install the amp I would just run the ground  to one of the seat bolts. Use sandpaper, dremel, or whatever you need to get the paint off of the floor at the bolt location, use a ring terminal on the ground wire and put the ring terminal between the floor and the seat track when you put it all back together. As for the power wire I have no clue as to where you can penetrate the fire wall easily on a Caravan. I usually try to penetrate on the same side as the battery to keep from having to cross the motor so I would suggest looking all over the fire wall for some form of rubber gromet. Once you find it cut a hole in it and run the wire through. I would also suggest looking at the firewall from the inside as well. Sometimes you will see the gromet from the inside but it is hidden behind something under the hood. I know on the 95-2003 or so Explorers there is a gromet that you can see from the inside but under the hood it is hidden by the brake booster.

Tips:

Fuse the power wire about 12" from where it connects to the battery, Before you start roll down all the windows and disconnect the + lead at the battery(well, pull the radio and test the blue/pink wire, then disconnect the battery), make sure all of the wires you run are not rubbing any sharp metal pieces, there are always a few in the dash at the radio location, make sure none of your wires are around any moving parts.

One last important thing.

If you are going to screw the amp to the floor in any way. First move the carpet or run your hand under it, alot of cars have wires running under the seat area and you don't want to drill through them. Also get under the car and see what is running under the floor pan, no sense drilling through a brake or fuel line.

I usually just liquid nail a piece of 1/4" plywood to the floorpan under the carpet and then just use short screws to secure the amp.