Don?t know if you?re still looking for an answer, but I see there are no replies.
I used this set up on my last vehicle. Haven?t done it in my Titan yet. The starter uses your factory remote keyless, or aftermarket alarm if you have one. It ties into the electronic lock pulse from the door locks. Hit the lock/arm button three times on your current remote and it will activate the remote start:
Maybe you can find a wiring diagram on this or the other forum. Haven?t looked yet myself. The instal is a bit tedious but not very complicated. Plus it can be removed if you decide you don?t like it in there.