As Jack said, there is not definitive answer. But I’ll try to give you a run down.
First, lets number the buttons/switches on the gun to make this easier.
1 - “Intensify” Button
2 - “Activate” Switch
3 - Top switch on the face of the gun (next to the bargraph)
4 - Bottom switch on the face of the gun (next to the bargraph)
5 - red push button at the tip of the gun
Switch “2” is used to turn on the lights of the at least the gun, if not the pack as well; as seen in the court room scene in GB2.
Switch “4” is used to turn on the vent light, as seen in the “heat 'em up” scene in GB1. Also in that scene, you see switch “3” in the up position. I would say this could be a safe bet as to the switch that controls the pack lights.
Now as to which push button makes the gun fire. I believe it is button “1”, the intensify button. Why I think this is because while Venkman and Egon are trapping slimer, Venkman takes his hand off the front grip but the flash bulb at the end of the barrel keeps going. So I assume that it is button “1” and not button “5” that fires the gun.
So to sum up what you can reasonably can deduce from the movies:
1 - “Intensify” Button - Fires the gun
2 - “Activate” Switch - turns on at least the gun lights
3 - Top switch on the face of the gun (next to the bargraph) - either not used or controls the pack lights
4 - Bottom switch on the face of the gun (next to the bargraph) - controls the vent light
5 - red push button at the tip of the gun - either not used or possibly controls the orange top hat light at the end of the gun.
Now I personally have all my lights turn on with one switch, as do others. So it comes down to how you WANT the lights to work.
Hopefully this helps you out.
