You are exactly right... it is simply a line level converter. It is spliced into one of the rear deck speakers. The small box takes the 15 or so watt level and converts it down to a low level that a Sub amplifier can accept. If you tried to directly imput the speaker level into an amp (without it's own converter built in) it would simply overload the amp and burn it out. I'm guessing you have a regular factory radio in you car and the previous owner added a sub in the trunk to make it sound acceptable. The RCA connectors can probably still be used. IF you leave them they are harmless and won't interfere with normal opperation.