well to make one you need a GPS and GSM modem.. Connected serially with a microcontroller and you have to code that microcontroller ( the circuit will must be having some extra electronics components too like capacitors and resistors )
its a long story whitout the prior knowledge of electronics and programming you cant make one..
You can start engine, stop engine, get the location, lock the car, start ac / heater JUST BY A SIGLE SMS
Like if you are in ur office in summer and cant bare the heat of ya car u can start the car about 10 to 15 mints before leaving the office just via an sms or a call 


