I'm trying to rule out an issue and was wondering if, with an aftermarket alarm installed, is there any possibility that the rear window defroster is connected to the same circuit as the horn or power locks? All 3 failed around the same time although the power locks still work, just not through the alarm system and I was only able to come up with an internal issue with the alarm for two of the 3 items since those 2 are connected to the alarm. I can't imagine why the defroster would be connected, but I wanted to check so I could at least rule it out.

 

It's highly likely that whoever installed the aftermarket alarm system could have tapped into the rear defroster's power for the alarm's power , and if the fuse for the rear defog went out it could take out the alarm too . The fuse called "power accessory" is always powered , and feeds the normally open contact of the defog relay .