And depending on which class 2 harness you get some either have an external chime, or the Scosche GMLAN2SR will allow you to change the volume of your chime using the dimmer switch on your dash. As far as not using a class 2 harness you will run into some minor issues. Depending on the platform you lose door chimes/dummy lights/keyless entry programming/dome lights. You do not have a 12v switched wire in your harness as it is computer controlled as well.