The couple times I used the adapter it was a matter of turning the SIRIUS radio off to listen to terrrestrial and on for SIRIUS. My son had a small toggle switch installed on his Tacoma.
Depending on your brand of car, it is just a matter of pulling the factory radio out just enough to get to the back where the factory antenna plugs into the radio. The Ford's I have used this product on required a small tool to release the clips holding in the factory radio. Any Best Buy, Circuit City, Ford house or car audio store has one of these tools. After you see it, you will say, " I could have made that!"