I always go to crutchfield.com when i have questions about car audio. You may give them a call or chat live with someone. I share some of your same questions and went there to look at some products to upgrade my infinity system.
You need an antenna adapter (40-cr10) . Second, standard harness will not work. You need Metra part # chto-02. And if you don't get sound you'll have to bypass the amp (above glovebox) with harness 70-6510, And unless your cutting the dash, a Double Din will not fit.
Might wanna read several of the threads we have on this as it is a common question.
Doing a double din isn't hard at all. Like I said, I make those bezels. When i said you could use a standard harness, yes, there are two. One is for the non infinity and is for the Infinity. Obviously, the one for the Infinity, which he has and would get, would power the OEM amp, turn it on and produce sound.
As for the antenna, you don't need anything special or nothing to change sizes or anything like that. You do need to lengthen your antenna and the best and cheapest way is to get a standard antenna from a junk yard. Run it from the head unit, behind the glove box and behind the pass side kick panel there is an antenna coupler that connects to the antenna mast. Disconnect the old and connect your new.
As for running a non Infinity harness, to do one better than that is to simply buy a 100ft roll of 12ga wire and run new wire to the doors and either to the back of the head unit or to the amp and leave all the small gauge OEM wiring alone.
Then all you need to do is tap into the OEM harness or cig lighter for power and ground.
Most guys who do this are running amps and like me, pushing 800 watts to the doors alone with thin 18 of 20 ga OEM wiring won't cut it.
Bypassing the amp with an after market head unit and stock infinity speakers will sound like crap because the speakers are frequency set because the OEM amp does it that way. In other words, the front 6x9s in the doors won't play highs, ect.