I am using a Scosche SLC-4 to convert my rear speaker level output to line level output. The convertor has a black wire sticking out of it which I assume to be a ground wire. Since it does not mention using the ground wire at all in the instructions, is it necessary to hook it up? If so, would I hook it up to the ground in the wire harness? Thanks!

http://www.scosche.com/scosche/electronic/art/SLC4.gif

I would guess that you would need to ground this to the nearest piece of metal that is connected to the electrical system. An existing screw that attaches to the car body is best.

Agreed.