The radio harness comes out of the radio and plugs into another harness right there behind the radio. If you look behind the opening above the cigerratte lighter (sorry, accessory outlet), with a little flashlight, you'll see 2 plugs. The upgrade plugs into the blue connector and the base into the grey. (this is visible as the car is being built. I haven't looked at a completed car that closely to see if it's visible).
The upgrade has 2 pigtails taped to the harness. Maybe 2-4 inches from the back of the radio. The base has 4 connection points (right,left,front,back) on the back of the radio.