Yes. Anything that is added to your car that is not GM will not be covered by the warranty. Also, any problems directly related to the alarm will not be covered. But only that which involves the alarm system will not, if the alarms system fails and burns up your wiring, Cadillac will not cover it. But if the transmission goes they still have to cover that. They can blanket void a warranty, thats protect by Federal Law. The Magnuson-Moss Warranty Statute. Btw-There is not fooling them, they can punch in the VIN and they know every option the car came with and if its not there, they don't have to cover it. Seems shady used car dealers will add options to cars taken from junk yards and the customer is none the wiser. When something goes wrong and Cadillac says the car was not built with those options, the customer is left holding the bad if there's a problem.
Just to many slip-shod installers out there. I would be very careful whom you chose to do this. Have you checked into weather Cadillac offers a accessory alarm kit? My 2005 CTS has one as standard equipment called the PASS-Key III Theft Deterent System. What year car is yours? You sure its not already part of the car?