A friend of mine has a 2002 Subaru Forester with what he says is the "high end" factory audio system. It has front and rear speakers(obviously) plus tweeters in the front doors, the 6 disk changer in the dash and a small sub below the driver seat. Well I was looking online for wiring harnesses and saw that none of them have lines for a sub, so here is my question: how is the sub wired into the system?