the wire for your starter motor is a big fat white with a black trace coming off your key barrel. (Or possibly black with a white trace) Its been a while so my memory is shit house.... just probe it with your test light and turn the key to start. You have to take the bottom part of the steering surround off to get to it. There are possibly other areas where you can get to the wire that will make it a shitload harder for a theif to get to, but this would be the easy way of doing it. You could use a relay and set it up, so that it takes a theif time to find it, and hide it in the engine bay somewhere. But then if he knows what he is doing he can start the car anyway. Slow down tactics are as good as it will get, Tow trucks can override any alarm.
I would immobilise the fuel pump, because if you start playing with the ECU and use your test light, there is a possibility you can blow it up.. You will need to probe for the wires though, cause I haven't touched them myself, so I don't know.
And the door sensor, is it an actual sensor?(I dont install alarms) Or does it just sense the power from the interior light switch? If its the latter I would try and get my hands on a wiring diagram, then you would be able to source power from a central point with the switches combine to turn on the light.
Sumo
P.S. im only a 1st year.... dont listen to me - No one else does lol