OEM Radio compatability? Antitheft codes

1. Are the plugs for the factory radios from the 8g and 9g the same as the 7g? In other words, can I just plug and play a 8g/9g radio into my 7g?
2. Anything special about the Infinity radios? On ebay I've seen write ups saying this radio is an Infinity system implying it might not work in all vehicles.
3. Any place other than dealer where I can get a antitheft code for free?

Just curious about all this as I need to replace my daily driver's radio and not interested in spending any real money on it. thanks,

Most dealers won't give out antitheft codes unless you own the vehicle. The person you are buying the radio from should have the code, if not stay away.