That is the right cable. The connector on the left in the picture should plug into your Phatbox, and the connector on the right into your Kenwood stereo. You should not have to do any splicing to get this to work.
Yet, you wrote that "it will NOT fit" your Phatbox. Do you mean the cable will not reach your Phatbox, or there is no matching connector to plug into in the back of the box? If it is the latter, can you show us a picture of the back of your Phatbox?