the viper remote start systems need to see ground in order for remote start and valet take over modes to occur. i typically recommend and install the black/white neutral safety sense of the viper system to the parking brake. this isn't 100% safe for manual transmissions, that's why the neutral position switch within the transmission is used to trigger the hood input of the remote start system.
when the vehicle is in gear the neutral position switch within the transmission will throw ground to the vehicle's ecu. you can wire inline with a diode to the remote start system's hood pin input. the remote start system will think the hood is open and will either turn off the remote start or prevent remote start from happening.
you can also wire the neutral position switch to a 12 volt relay to send ground to the viper's neutral safety sense input. wire as follows:
use the 2nd ignition output of the remote start system (this wire isn't used for the impreza since it only has 1 ignition wire off the ignition harness) to Pin 86
Wire the neutral position switch to Pin 85. It's advisable to install a quenching diode to prevent voltage feedback to either the ecu or the neutral position switch. A pre-wired starter kill will typically come with the diode installed on Pin 86 and Pin 85.
Parking brake wire to Pin 87a
Pin 30 to remote start systems neutral safety sense input (ie. black/white wire for dei remote start systems)
This will provide at least 2 levels of safety, the neutral position switch and the parking brake check. If the vehicle is in neutral and the parking brake isn't engaged the neutral safety sense will never see ground and thus preventing or disabling remote start/valet take over.
if the parking brake is engaged and the vehicle is in gear, the relay trips open and the neutral safety sense never see's ground.
Always use common sense anyway b/c the neutral position switch is mechanical and may not throw ground at all.