I've been using one of these for years. It has a 3 prong plug because it has no microphone and runs off AC or a pair of AA batteries that you put inside. You can also set it to broadcast on any FM station frequency.
I bought the $30 radioshack special. Works great as far as pushing the sound to your radio. It has auto tune etc etc. I just didn't see the point in putting a lot of money into something I don't use but a handful of times a day. They have wired and wireless ones with controllers.