Smitty025-
I highly recommend that you read the Avital 4113 manual (link in previous post) before you begin. It offers better insight than the newer Ready Remote manuals. As for your questions...
H1/4 (-)Activation Input wire--This wire is an optional input. Any time this wire sees a ground, the remote start will attempt to start the vehicle. Normally this wire is left open with the end taped to prevent any inadvertant contact. Teambaja used this wire to test his installation. Application for this wire is to use the remote start with a switch in the vehicle, or to use the remote start with a remote cell/paging system to start the car. It could also be used for test of the installation.
H3/1 Neutral Safety Input--You are correct...This signal is required to be grounded for the remote start to operate. The intent is to prevent Remote Start from occuring when the car is in gear. If your car cannot start in gear, as most automatics today, this signal is just grounded. If the vehicle can start in gear, this wire must be connected to the neutral start switch in the vehicle; this prevents the vehicle from remote starting unless in Park or Neutral. Also, a switch is usually provided for use in series between the wire and the ground. The intent is that when the switch is open, the remote start will not operate, as the Neutral Safety wire does not see a ground. When the switch is closed, the unit will be able to start as normal. The application of the switch is for a "Valet" mode, used when having the vehicle serviced, etc. For a 2002 Sentra automatic, this signal can be grounded through the supplied switch.
H1/9 Light Flash Output--This output activates any time the unit is used to lock or unlock the car, or during remote start. The output can be wired to either your parking lights (common in the US/Canada), or your hazards (common in Europe). Your choice. Whichever you choose, make sure the polarity shorting plug on the side of the unit matches your vehicle's signal polarity for the chosen lights. If I recall, Ready Remote's default is Positive (+). For a 2002 Sentra, you can pick up positive parking lights in either the driver's kick panel or at the light switch. Remember to test every wire with a volt meter before connecting.
H3/5 200mA Status/Rear Defogger-This signal has 2 functions; Factory default is Status output-- This provides a ground anytime the vehicle is running. Typical application is to enable an immobilizer bypass, used to start the car if you have a "Chipped" key. If you don't have an immobilizer, or if you are not using the Status output (only one Status output on the Ready Remote as I recall), the output can be configured as either a 10 minute constant output or as a pulse (Depends on how it is wired into the vehicle), that would be used to turn on your defoggers in your car. For a 2002 Sentra, you may have an immobilizer (some models did), and if so, will need a bypass. Use this status signal to enable your bypass.
Hope this helps.
Lectric Guy