Remote start w/factory keyless, 03 Sierra

Yes, the 457G will do bypass of passlock, alarm interfacing, and door locks. Here is the installation manual if you would like to familiarize yourself with it before purchase. 1. You could use the factory remotes to activate a starter yes, but the factory remotes have very limited range. The factory keyless will remain completely intact and will coexist with the aftermarket system with no complications at all so long as the installation of the aftermarket system is done properly. 2. You would need to bypass the passlock II system no matter what starter you are installing. I recommend just picking up the 457G or a GMDL-BP from bypasskit.com which will do most of the interfacing for you with only one connection to the OBDII Plug and the rest made to your after market system.

If you want to have only one remote, here are your two most popular options: 1. Buy a remote start that works off the factory remotes (Audiovox AS-9075A, DEI Valet 561T), and a bypass module for the truck's Passlock II anti-theft system (Audiovox AS-GM4, DEI 555L or 556L). The factory remotes have limited range. You'll be fine from a single family house, but probably not from a mall, supermarket, or your workplace. Try the factory remote from where you plan to start the car. If you can't operate the locks from there, then don't bother going further. 2. Buy a remote starter that comes with its own two remotes, including doorlocks and a trunk-release output. (Audiovox Prestige APS-686T or APS-687 is good.) Buy an all-in-one bypass module that'll control the doorlocks, panic, and Passlock II bypass. The DEI 457G has many shortcomings; avoid it. From bypasskit.com, the GM-DLBP is excellent and will do all that you need. The XK-01 is more expensive, but will also activate the rear defrost, heated mirrors, and heated seats when it's cold out.