I'm interested in adding an under-seat subwoofer to my stock stereo. Originally, I was planning on using a line output converter to create an RCA output. But then I thought I remembered that there is a factory Subaru sub available. Although I am not interested in that model, does it mean that there is some type of subwoofer signal coming out of the factory headunit. Or better yet, is there a power and signal source wherever the OEM sub mounts?