1992 Corvette

I am new to car audio so please pardon any ignorance found in my questions: I recently added a self Powered sub (memphis) to my aftermarket radio (kenwood) The sub is an enclosed unit I have power at the box. when I plug in both RCA cables I dont get any sound, however I can unplug one of the cables at the box and the sub sounds fine (to my untrained ear). I was just curious as to what might be causing this. Any answers would be appreciated Thanks