Best Buy sells the Kenwood compatible receiver. I think its KDC-903. You can't go wrong, its the only kenwood specific one they'll have, it cost about 80 bucks. Its a plain black box that says "Kenwood" on it. You don't want one of the universal units since you already have the Kenwood Sirius compatible head unit. Only the Kenwood reciver works, though.
This come with a wire that just plugs into the back of your radio and the box is mounted in your car somewhere. Mine is in my trunk, with the wire run under the passenger door trim. You'll then plug the antenna into that receiver. Mine runs up the arm/hinge thing that supports the trunk and out, with the antenna stuck on the trunk lid. Its a magnet. I installed it myself, its pretty easy or they'll do it for relatively short money.