the "door sensor" is comes with is a hood pin. theres a shock sensor as well, but everything else is optional and must be purchased seperately.
the "door trigger" is one wire. it's tapped into the factory harness, and recieves a pulse when any of you're doors are opened (doors have their own pins, and activate the dome light when they open...rear doors often do NOT have pins OR a dome like trigger)
and, unless you're **** comfortable with rewiring parts of your car, get it professionally installed. i installed mine, but i also have been doing them for a couple years (on the side)
my suggestion, dont buy it yourself. if you're going to have it installed, have a shop sell it to you and install it. it'll be a bit more money, but everything will be warrantied (parts and labor)
*EDIT
DO NOT BUY REFURBISHED ALARMS, EVER. IF IT BROKE ONCE, IT WILL BREAK AGAIN
