Here is information to help anyone who is thinking about upgrading the "Premium Audio System" Navigation (and Non-Navigation) stereo in their Civic EX & EX-L Coupe and Si Sedan/Coupe. The "Premium Audio System" featured the external 360-watt amplifier with 7 speaker system, including a subwoofer. The premium audio system is pretty awful. The subwoofer and amplifier are useless. The amplifier is flawed because it has the frequency crossovers built-in for the front tweeters, rear speakers, and subwoofer. If you replace the speakers and keep the factory amplifier, you still get poor sound because of the internal crossovers in the amplifier. The solution is to remove the factory amplifier, speakers, and the subwoofer. The best part of the premium system is the head unit, because it is unpowered with pre-amp level outputs for the front, rear, and subwoofer. Most posts I read about stereo upgrades recommended a line out converter, even on the premium audio system, but an LOC is not necessary with the premium audio system. Since the head unit is unpowered, you do not need a line out converter. All you need to do is splice the factory speaker output wiring and convert them to RCA plugs to feed directly into an aftermarket amplifier. You can do this for the front, rear, and subwoofer outputs from the Premium Audio System head unit (Navigation and Non-Navigation). You do need an amplifier that is compatible with differential balanced outputs. JL Audio and Infinity make amps that accept differential balanced inputs. This type of stereo upgrade is even easier by utilizing the 24-pin connector at the factory amplifier because it has the pre-amp level speaker outputs, subwoofer pre-amp level output, and amplifier remote turn-on lead from the Premium Audio head unit all in one harness. This is the harness you can use to splice the speaker and subwoofer wires to RCA plugs to feed directly into an aftermarket amp, including the remote turn-on lead. You can also use the 18-pin connector at the factory amplifier to feed those wires to the aftermarket amplifier speaker outputs. Then use the factory wiring to connect new speakers in the factory locations, and run a new wire to the aftermarket subwoofer installed in the trunk. This was the type of upgrade I had done in my Civic EX-L Coupe with Premium Audio Navi. I wanted to keep the Navigation head unit because it offered all the features I needed and I wanted to retain the factory look and retain the audio information on the i-MID screen. I had installed stereos before when I was in college, but I did not have the experience to build a subwoofer box. I also wanted to have a professional shop do the installation, one that was familiar with Civics, knew how to do custom work, and a clean installation. The JL Audio XD700/5 amplifier is perfect for the Civic. It is a 5 channel amp that accepts differential balanced inputs, and fits nicely under the passenger seat. It also has a subwoofer input. It offers plenty of power at 75w RMS for the front and rear, and 300w RMS for the subwoofer. The shop removed the factory amp and converted the wiring to RCA plugs and fed the pre-amp level speaker and subwoofer outputs directly into the amp. I can use the factory subwoofer level adjustment to control the level of the aftermarket subwoofer. The front and rear speakers are the Focal Integration series. The front speakers are 6 1/2 inch components and the rear speakers are 6 1/2 two-ways. The factory subwoofer was removed from the rear deck. A custom subwoofer box was built into the side of the trunk for a 10" JL Audio W3 subwoofer. I did not want to lose trunk space. The rear deck and entire trunk was lined with dynamat. Additional dynamat was used around all the speaker mounting openings. The shop did an amazing job with the tweeter installation in the front pillars, and the custom subwoofer enclosure in the trunk. The door and rear deck speakers are under the factory grilles since they are entire panels. The sound is incredible and everything works as intended. The subwoofer really rocks with clean, solid bass throughout the vehicle. Not boomy at all, which was what I was looking for. I listen to rock, classic rock, alternative, 80s alternative/new wave, jazz, new age, and a few rap hits here and there. The same upgrade can be done with the Non-Navigation Premium system. Here is the link for the wiring diagram for the Non-Navigation Premium Audio System. (At the end of this post, after the pictures, is the wiring diagram for the Navigation head unit and factory amplifier. It is attached as a PDF file.) http://9thcivic.com/forum/threads/2012-civic-audio-wiring-guide-pinouts-for-factory-radio.5916/ The 24-pin A-connector is the same for both the Navigation and Non-Navigation Premium head units. The E-connector is different, but the subwoofer wires are the same color. The Navigation E-connector is a 16-pin connector and the Non-Navigation E-connector is an 8-pin connector. The pre-amp level subwoofer output on the Navigation unit is E3 Pink and E11 Blue for Sub+ and Sub-. The pre-amp level subwoofer output on the Non-Navigation unit is E5 Pink and E7 Blue for Sub+ and Sub-. The external amplifiers are the same for Navigation and Non-Navigation Premium Audio units, so you can use the A and B harness connectors at the amplifier to splice into the factory wiring to feed an aftermarket amp. If you have a Non-SI sedan or LX coupe with the standard 160-watt audio system, then both Navi and Non-Navi systems have a powered head unit with high-level speaker outputs, and no subwoofer output. Those systems require a Line Out Converter for an aftermarket amp, or an amp that can accept high-level speaker inputs (most do, including the JL Audio XD700/5). The Premium Audio System has the benefit of having pre-amp level outputs and the subwoofer output, making an upgrade very easy. The shop loved having the wiring diagram because it was dead-on accurate. Thanks to this board for posting the wiring diagram for the external amplifier, it was a big help! If anyone is in the Southern California area, then Santa Clarita Autosound is the place to go. These guys did an amazing job with the installation. I am very happy with the work they did, and they are highly recommended (even on Yelp). Now the "Premium Audio System" tagline on the head unit actually means something. Factory speakers, factory amp, and factory subwoofer removed from the Civic. The speakers were awful. At least the front speakers had much larger magnets. Explained why they were so much louder than the rears. The JL amp installed under the passenger seat. Front passenger tweeter installed in the factory A-pillar. The driver side tweeter installed in the factory A-pillar. Close up of the tweeter installed in the factory A-pillar. Custom subwoofer box built into the side of the trunk. 10" JL Audio W3 subwoofer. The custom box was built to specifications for the JL Audio subwoofer, and I still have all of the trunk space for cargo. Custom amp wiring under the hood. The shop wrapped the wire in plastic tubing for protection, and to retain the factory look.