Replacing mach 460 with kenwood system

[slug12345]

I recently bought a 94 Mustang GT Convertible.

Previous owner had replaced the original stereo with a JVC, everything else was still mach 460 system. Not sure what they did wrong but the only speakers working were the door and back seat tweeters. I recently replaced the JVC with a kenwood and installed kenwood door and back speakers. Still the only speakers that work are the mach 460 tweeters. I bought everything from Crutchfield so I followed step by step instructions for installation. From what I have read I probably need to bypass the mach 460 system but I'm not sure how to do this. Maybe its time to take it to a professional. Any ideas??

 

[slug12345]

Wiring Adaptor

I've got the wiring adaptor from crutchfield (120705510) which is suppose to give me the ability to integrate the kenwood stereo and speakers into the mach 460 system. One wiring harness is gray rectangle which I got it hooked up but the adapter for the speakers is sqare and black and I can't figure out what it plugs in to? Am I just not looking in the right spot?