most installers will put the remote start module near the steering column and with the way the LS is built, the knee bolster panel comes off and gives plenty of room under there so that is where i would start looking. at a minimum they would have to tap into the ignition wires at the key cylinder, so take the knee panel off and follow the non factory wires coming from the key cylinder all the way to the module. you could also follow the antenna wire if the antenna is mounted somewhere on the windshield.
but as Joe mentioned, just unplugging the RS system may leave the car unable to start if the system has an alarm or starter kill (which is an option for all compustars systems) if this is the case, all compustars use an external relay for the starter kill, so you could just unplug the relay from the socket and run a jumper wire from terminal 30 to terminal 87. however with the LS, there should be a couple of extra relays under there for all of the extra ignition wires the LS has, so you would have to know which is the right relay, one way to find out would be to unplug those relays one at a time (while the RS is still hooked up) and try to start the car with the key, once you remove a relay and the car stops trying to start, you have found the starter kill.
proceed with caution
as far as the codes go, do they only happen when using the RS?
do they pop up when starting with the key after they have been cleared?
most remote start problems that throw codes only do so when using the remote start, if it ios happening from using the key, it very well might not be the RS system.
BUT (not sure of the exact code) ive seen a CEL code for something about the fuel level sensor when not all of the ignition wires are powered up (you have to wire in an extra relay that is not common and not included in the remote start box) but you will only get this code from using the remote start, never from using the key. if this is your problem, all that you need to do is have an extra ignition relay added to the remote start system (any compustar dealer should be able to do this for around a half an hour of labor plus the relay)
