Need help replacing head unit in a '03 Mach 460
 

Last night, I tried replacing the stock 6CD changer head unit in my '03 convertible with a Mach 460 system. I've done this many times before in other cars, and didn't think this would be any different.

I ordered this harness:
http://www.crutchfield.com/S-kc8XnJT...sp?i=120705700
from Crutchfield and wired everything up as best I could. One of the first things I noticed was that there were many more wires on the factory harness side than on the Crutchfield harness.

When I turned everything on, all I got was sound from the tweeters. I tried hooking things up both with and without this Scosche speaker level converter:
http://www.scosche.com/scosche.aspx?...&CategoryID=36
and found that I got better sound without it.

The Crutchfield harness had two wires marked as subwoofer + and -. Splicing these into one of the speaker channels only got bass from the rear subs, and a pretty healthy hum as well. When I grounded the wire marked subwoofer ground, the bass died completely.

At this point, I gave up and went back inside before my wife thought I had snuck out on her. But I would really, really like to get this working. How do I need to wire this up to get it working, without the hum, and using the factory amps? Is there a better harness I can buy?