Just used a line converter and tapped into the rear door speakers for my sub amp.

just saw your post today with all of your mods, abnd it looks great! I am getting my stereo system installed soon and was wondering if you used a signal summing device or anything along those lines. I have always just tapped into the speaker wire for signal but I heard the V's Bose sub lowers its signal when you turn it up, hence the need for some outside adapter???