I have not worked on a Fiat, nor have I seen a Falcon alarm, but I will try to help.
--As far as the indicator wires, those would be the left and right turn signals, correct?
Open the trunk, in the back of the car, and move the carpeting or panels so that you san see the backside of the taillamps.
Most likely, you will see that all the light bulbs back there have the same color for one wire.........that is probably ground or earth........and then, each bulb will have a different-color wire.
Pay attention to which bulb is for the turn indicator, and make a note of the color.
Now, go inside the car, to the wire bundle under the carpet, on the floor.........find the same color wire.......TEST THE WIRE with a multimeter or test light to verify that it is the turn-indicator wire.....and there you have it.
---Door trigger:
If you are going to combine the car's left and right door pinswitch wires, to connect both of them to the alarm......you should be using diodes to isolate them from each other........Fiat intended them to be separate for a reason......simply tying them together with wire will alter the design of the car.
At this the12volt.com website, on the left under the "Basics" section, is an excellent article on how and why to use diodes.
However, since ALL of the doors turn on the interior domelight, you may be able to simply use the domelight wire as your door trigger.
First, unscrew the domelight from the car's roof, and let it hang by its wires.
Next, TEST both of the wires at the domelight, and determine which one changes state when the door is opened and closed.
Then, look for the same wire at a more convenient place in the car.....usually you can remove the trim from the driver's-side A-pillar (windshield post), and locate the wire there.
If all else fails, it would only take a few extra minutes to run a wire to the domelight itself.
----Shock sensor:
Most likely, the red is for positive power, the black is for negative ground/earth..........and the green wire will put out a ground when the sensor is triggered.
After you read the section about diodes, you will learn how to connect multiple triggers to your alarm, even if it has only one trigger input.
You can use the diodes to isolate the shock sensor, and the domelight, from each other........then combine them and connect them both to the alarm's door trigger input.
One downside to all this is that even a small bump to the car is going to put the alarm into full-trigger mode with lights and siren..........you will not be able to use the shock sensor's "warn-away" feature.