Door won't open from the outside on your CTS? There's a good chance the micro switches in the door handle need replaced. This video will show you how to test the micro switches, and how to replace them if they are bad. This video will also help if you need to get the door panel off for other reasons. This repair was done on a 2008 CTS with the key-less ignition option, on the passenger side front door. I suspect that the drivers door process would be similar. For the passenger side door (right side) you will need GM part number 25864472 (HOUSING K) if you determine that the micro switch(es) are bad. I purchased mine from gmpartsdirect.com - Have fun!
Disclaimer: I work on computers for a living, NOT Cadillacs. This video is for informational purposes only. I am by no means an expert. Please take any advice or information from this video at your own risk. I accept no responsibility if you break something, or hurt yourself, your vehicle, or others.