My wife has an 05 Cobalt LT. The horn does not work and I think it is related to the next problem. When you unlock the door or the truck (with the key, we have no remote) the 4 ways flash for about 15 seconds. I think the horn was disconnected because of this. I have no clue why they do this. Any Ideas? My wife wants her horn back, LOL.
That's just the way the alarm works (unfortunately). If the alarm is armed and you unlock the door with the key the alarm goes off. If you don't want the alarm to go off you have to use a key fob to unlock the doors (you should be able to get one from the dealer). Or you can manually push down the door locks which will not arm the alarm at all.