This is for the 2004 Galant GTS, but should work for other models respectively.
I have the 6 speaker 6 cd Infinity sound system and this will allow you to add a sub and amp without modifying the existing speaker setup or modifying the stock amp; which is located on the passenger side to the right of the glove box almost touching the wheel well for anyone wondering where its at.
You can see here the engine compartment and plently of places to install a fuse and clean wiring.
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To the right of the airbox gives you access to an open area which the main electrical and computer connections go through which has plenty of area for a 4 gauge wire to run through.
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If you open the driver door you can see where the wires turn into the car. This also give you access to make installing the power wire easier.
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By getting a wood dowel and lightly taping the 4 gauge wire onto the tip you can feed it in through the gap available at the front of the door and grab off the wire at that open area next to the air box.
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The just connect up your wire to the battery and don't forget the fuse.
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There was a piece of plastic next to the battery and I just zip tied the fuse onto it.
Now I just fed the 4 gauge wire into the car using the same hole that the rest of the cables used. You can either go under the edge of the rubber or you'll have to cuz a hole into the rubber otherwise. The edge of the metal was not sharp so I just fed under the rubber gasket.
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Then I routed the cable under the quarter panel trim pieces.
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While you have the quarter panel trim off you can run the remote wire along with the power wire. To devise a remote wire that turns the amp on when the car turns on since you can't tap into the radio I used the fused side of the cigarette lighter. I figure it is the least used circuit and will have the least power draw so the amp has decent power going to it to turn it on. I used a wire tap so I didn't have to cut the wire, seems the easiest route to take. The (Blue wire with small white line) is the power wire for the cigarette lighter.
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There are clips that you can use to secure the 4 gauge wire and the remote wire, don't even need zipties.
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Then to get the wires to the trunk you can use the dowel again and pull the wires through underside of the back seat.
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Now to get the sound I just tapped into the back left speaker using a tap that converts to phono out for the amp input. You need to drill a hole infront of the speaker to gain access to the speaker wires.
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Then tap into the wires and tuck the wire taps in the little area infront of the speaker.
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Then using double sided sticky tape just stick the audio tap onto the back side of the seats and ground the audio tap.
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And then just connect up your wires and your good to go.
IMPORTANT!!!
You'll need at least 25 ft. of 4 gauge wire to reach from the battery to the trunk with a little to spare. 30 ft if you want to put the sub(s) on the right side of car.
Happy Thumping