I have been looking everywhere and I can't quite figure out where the speaker wires are (or amp turn-on wire for that matter) that I am looking for to connect to my amp to ultimately give a signal to a subwoofer. Since my car lacks the Bose system, there is a clear, unfilled slot for the subwoofer in the trunk so I assumed the speaker wires would be near this but I can't get my hands on a wiring diagram or anything and all the guides people have posted have been for cars with the Bose system installed. Any help would be appreciated, thanks. I have included pictures of what the roof of the trunk looks like if that helps.

 

https://www.nicoclub.com/service-man...F2008%2FAV.pdf
Below that window you can "click here to download a copy".

If I'm reading this book right, your system has a 4-channel amp built into the head unit - one output channel for each door. It seems that all the '08 sedans shared the rear shelf cutout for a sub (which is handy) but you likely won't find any sub-related wiring already in your trunk. You'd need to tap off of the door speaker outputs of the head unit for signal. And for an amp-on signal you might be able to tap off the antenna amp in the driver's side C-pillar. The satellite receiver already in the trunk appears to be always on with the ignition so that's not ideal. Or just run a switch controlled +12 from somewhere.

Hope this helps.

 

Would it be possible or ideal to tap directly off one of the door speaker wires within the door itself closer to the speaker or should I go as far as removing the head unit to tap the wires off of there?

 

There are much more knowledgable car audio folks in here ... but I assume many sub amps have "speaker level" inputs and process the audio with a low-pass filter to drive the subwoofer? If thats how they work I don't think it would matter where you grabbed the signal. The ultimate easiest might even be to pull back the rubber boot going into a rear door, figure out which wires to tap, and grab them right there... I also don't know if you'd need both left and right channels or just one (seeing as a sub is usually mono). Of course the sub would then follow the volume you're sending to that particular door. Like if you fade all the sound to the front but have tapped a rear door, no more sub. But I think that's all doable and you could probably balance it to sound ok. Hang on for a couple more opinions in here from guys who really know what they're talking about.