1990 Mercury Marquis Alternator/Battery problem

The battery is completely charged and I just replaced the alternator and voltage regulator. There is a part on the front of the battery and behind the right headlight, a box that has wire's going to it labeled bat, alt, w/s, and mod. When you put hte negative terminal to the post while the positive is connected is clicks. My question is what is the name of the part that is connected to the battery that is labeled? And is the clicking normal? The car will die while driving down the road at 45 mph, and i've had battery and alternator tested after replacing htem and the car die's when the positive terminal is removed from the post. Could that "box" be the reason the car will die as in the battery and alternator are connected thru that piece and no charge is going thru?