Save money and install your own car stereo. Part two, subs and amps.
This is part two of my DIY car stereo install. You can find part one, installing a head unit, here.
If you hooked the negative battery terminal back up, or if you are starting from here, unhook it again. Now comes the fun part. The big red wire needs to go from the engine bay, through the firewall to the inside of the vehicle. Where and how you do this varies from car to car, Sometimes its nice and easy, sometimes its not. What you are looking for is where the wires for the ECU go inside. I've found this on the back of the firewall, on both drivers and passengers side, in either wheel well, and sometimes you just can't get to it. If you can't get to it, You'll have to drill a small hole for it. If that's the case, find a spot that has nothing on either side, and make sure you just drill enough to punch through, you don't want the drill bit going a couple inches inside the car, because chances are if it does you'll hit wires or something else.
Once you have the power wire inside the car, you'll want to run it on the opposite side of the RCA's. Take off the kick guards on the bottom of the door rail, and there should be a group of wires going to the back of the car under the carpet. Run the wires in there all the way back.
Once you get to the back seat, run the wire either tucked in the side of the seat, or take out the seats and run it under. The method really doesn't matter, either way the wire will stay hidden, you just need to get it to the trunk. After the power wire, RCA's and signal wire are in the truck, you need to find a place for the ground. Under the trunk carpet, on most vehicles, is nothing but metal. Any spot is fine. If the metal around the bolt is painted, use a wire brush to remove the paint so the ground is against metal. If installing multiple amps, use a splitter instead of running more than one wire. Hook all the wires into the amp, take the fuse out of your power wire, and hook the power wire up the the positive. After the power wire is hooked to the battery, put the fuse back in.
4 channel amps
If you're installing a 4 channel amp, its time to run the speaker wire for that. Since it's already in the trunk anyways, you can just run new wires to the rear speakers to make it easier, and just leave the wires from the factory harness unhooked up front (rear speakers should be the green and purple from the harness). Next, run two sets of speaker wire from the front outputs of the amp, to the head unit. These wires should be white and grey, with white being left and grey being right. Hook those up using the same connectors from part one. After that is done, test the head unit using the same method as before.
Because of all the variations between amps, type of subs, and number of subs, it's difficult to say exactly how to wire up the subs for every situation. You'll have to look at if your subs are dual or single voice coil, how low your amp can still be stable, and how many subs you have. Crutchfield has a really good guide based on all of that. This didn't used to be the case, but most amps these days are at the very least 2 Ohm stable. If you are using a 2 channel amp for 2 subs, I have found that it is usually better to bridge the amp, it should say which to connectors to use for bridging, than to have one to each. There are 2 main ways to wire up 2 subs to a single amp, series or parallel. For series, you'll have the negative of the amp go to the negative of one sub, the positive of the amp to the positive of the other sub, then the other positive and negative of the subs connected. For parallel, both the positive and negative of the both subs go to the amp.
A piece of warning, don't wire you amp lower than what it is stable. It will work, and it will be louder, but your amp will over heat, and quickly.