So please tell me how you plan on hooking up a sub to a stock headunit. I'm pretty sure you need an aftermarket headunit for hondas. I've hooked up 3 car sound systems and have never been able to use the stock headunit.

 

^Tap into the rear speaker wires with your RCA cables and use an amp with a built in low-pass filter.

 

have you tried to follow the wires to see what it goes to?

 

Got one

 

It appears the wires hooked to it run from both the speakers and the lights, and one line runs up to the battery

 

I am almost certain it is some kind of grounding device. Probably there to filter noise out of the stereo system to reduce static, etc.

 

That sounds like the most likely, now that you mention it...

 

it's a coil assembly for the glass antenna