This is the one that I bought and it works great:

Amazon.com: Scosche FM-MOD02 FM Modulator for iPod, Satellite Radio or Portable Music Player:…

You plug it inline with the antenna (like the other) with the GM adapters then you run your power for the switch. It has a couple of channels to choose from that it simulates to give you your hookup. It does not take out the functionality of your radio at all even when it's on, only takes over the one station, I even have XM hooked up before this thing.