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

It sounds like you have a short in either one of the cables, or one of the RCA jacks.  Try running a different set of RCA cables temporarily to see if that helps.