Call me stuck in the 80's or whatever but I want to be able to listen to cassettes in my new GT (I have a TON). Ford makes a radio for the F150 that should swap right in. I did the same on my 2004 F150 (single CD for CD/Cassette). But the Shaker 500 CD6 that is in my Mustang has an additional plug on the back (the smallest one on the left next to antenna hole). I haven't actually removed my radio to see if this used (got pics off ebay). So the question is, what is this plug in for? My car does have the active anti-theft system, does have to do with that? Anybody else done this?