Basically, you are going to run the antenna from your tuner to the back of the car and connect it to where the SAT antenna terminates in the trunk.
I installed the OEM XM tuner, but its basically the same logic, this is the PDF for the install, it will show you exactly where the connector is.
http://www.trdsparks.com/install/PT546-02080inst.pdf
The connector in the car is square. Your connector may be different. If so, you can either splice the wires together, or pay $60 for an adapter, both will yield the same results.