[geoff4376]

Started happening randomly yesterday. The door was locked (I'm not sure how it got locked), I unlocked it with the key, and as soon as I opened the door, the alarm started going crazy.

I believe it is an aftermarket alarm, because it wasn't just the horn that was going off, but a much louder alarm noise. There is also some sort of sensor/antenna thing on the inside of the windshield that says "hx" on it. No clue what it is, since I bought the car used.

I ended up disconnecting the battery to make the car shut up. I tried the whole reconnect the battery, turn the key to on, and wait 30 seconds thing, but that didn't do anything. The alarm randomly stopped, then started again after ~10 seconds.

Possible variables: the hood was open when I unlocked the door initially, I had the key w/remote in my pocket, doors may have been locked from the interior (honestly no clue how they got locked).

What can I try next?

 

[geoff4376]

Found a Avital Avistart 4100 under the dash. Disconnected that, plugged the 1 plug from that back into the ignition.

Now the alarm isn't going off, but I have a new problem. When I try to start the car, it won't start, but it will keep trying to turn over until I turn the key to the off position. Like if i let go of the key from the start position, it will keep trying to start until I turn the key off.

 

[avisitor]

Sounds like the wiring isn't quite right.
You are going to need a schematic for your vehicle

 

[flemmons]

Also, be sure the battery is good, and all grounds are clean and tight. Had an '06 without aftermarket alarm do the exact thing you describe. Turned out to be a bad battery cable clamp. The voltage drop across the bad connection drove the factory anti-theft system crazy.