Adding a sub and amp???

[MACH460MUSTANG]

I have a 2000 Mustang with the 460 system (obviously by my name) and I attempted to add a Kenwwod amp and MTX 12" sub to the system. It didn't work. I even took it to a well-respected car audio place in town and they called all over and everybody said that you can't add any kind of in-line converter (need that for lack of RCA jacks on radio) or the whole stereo shuts down. Any info would be greatly appreciated, especially since I have already bought everything. It sounds good now but I would like that extra deep bass for certain music. HELP!!