Rear Speaker Installation

HELP! i'm a "rookie" when it comes to car audio installation. I recently purchased some Infiniti 6.5 inch speakers to install as rear speakers. After going through the hassels of ripping out the rear seat to access the speakers, I successfully removed the old stock speakers. I installed the new speakers, but the connection is not compatible. On the factory speaker wires they had a small black box that had the positive and negative ends sitting perpendicular to each other, that plugged into the speaker. I broke apart the box to access the positive and negative cables and attached them to the proper ends of the speakers, but one small problem... no sound! I ensured that both ends are connected correctly, but still no success. Well, I got into my car and started it up to back it out of the driveway and park it into the street. As soon as I turned the ignition off the rear speakers came on, but their sound was kinda distorted. Basically, the speakers only work when I hit a bump on the road somtimes. I'm guessing this is a wiring problem? The local shops want a buttload of money just to check it out. Any ideas on what the problem is? I've heard from some local service techs that infiniti probably has an amp going to the speakers thats causing the problem. Can someone offer me some much needed advice here? THANKS!