So i just put an aftermarket alarm and remote start in my Jeep here is what they have to do if you have the centry key deal....
You have to go to the Dealer and purchase a Blank key dont have them cut it as it will not be nessary. They will just program it so that it works with the system. The then hook up the alarm/remote start and take that key put it in a box that transmits the code up through a wire to teh ignition spot. This sends the signal that there is a key in the ignition and then the jeep starts.
They way mine works is I have to then take a real key (the one I normaly use) and turn the ignition to the on spot and then hit the break. once I do this then the engine says running and I can go.
However if I do not put a key in the ignition and hit the break to shift etc it will kill the jeep and I will have to start it normaly.
Nice thing is that it works in reverse also I can be running the jeep click the remote start button and then take the key out it stays running and locked while I am in the Circle K getting a soda etc.
In tucson with 100 degree temps this is quite handy.
One thing of caution make sure you dont start yoru jeep in your pocket
I park near my office and I have come out to my jeep running and a quarter of a tank gone who knows how long it was sitting their running

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Later,
Dan