In the Walking Dead episode "This Sorrowful Life," Merle hot-wires what looks to be an older car. The car had a car alarm that caused a walker attack. I don't recall there being car alarms on older cars such as this car. Would it have been a reasonable expectation for Merle to think the car might have an alarm? Did cars like the one he hot-wired come with an alarm?
Older model vehicles such as the one Merel hot wires did not come stock with alarms but if you look at the upgraded wheels (22"?) and upgraded stereo then it's safe to assume the owner also installed an alarm. I'd imagine he didn't think there would be an alarm.
However, the whole idea of this car sitting for 1yr+ and still starting is ridiculous. It would be unreasonable for Merel to think that that the car would even start. It's even harder to believe that he was able to hot wire it with a single hand (cutting the wire, stripping both ends and shorting them).
Also, aftermarket alarms usually do not draw power from inside of the cabin so he would have needed to cut the power from under the hood.