Okay, the idea is; you get in the car, put the key in the ignition, turn it to IGN, type in the code, and have the car start. Without the correct code, the car won't start. If the code entered is incorrect, it'll trigger the alarm.
So basically, the ATTiny2313 has to trigger the output (ie, the starter relay) until it recieves an input from the ECU stating that the engine is started, so that it stops triggering the starter relay. If the input is recieved without the code having been entered, output #2 is triggered (the alarm, containing a GSM & GPS module to text cars status and location at 10 minute intervals).
It's also got to drive a 2x16 character LCD - in order to relay messages, and *'s whenever any number is entered on the 4x3 keypad.
From what i've seen on the datasheet, i have another two I/O's at the very least in which to accomplish this.