Remote Start for an 03 Sierra

I recommend any DEI system.  I have used them in both my cars.  My beater car has the Avital 4000, which is just a basic remote start, but has aux option tied to keyless entry, and my summer car has the clifford rs3.5x which is remote start, keyless, and alarm with 2 way paging remote.  both made by DEI.  Both are super easy to install and have built in relays that will work with most aplications.  As far as keeping your factory remotes, forget it and install the whole package with keyless becuase in most gm's that i've worked on, the factory keyless system will turn off once the vehicle is running, So you'd have to use the key to unlock your doors after starting the car with your remote.  If you go with a DEI system, you will need a 555U (or 555L, can't remember exactly) to bypass the passlock system.  All DEI remote starts have primary and secondary ACC hookups which will turn on your blower motor, and have the option to set an aux to turn on your rear defroster too.

Hope that helps somewhat.