Well, I finally found a nice quiet spot in a seaport and plugged the box in. My horn started beeping on and off for 30 seconds. So, it looks like it's the brainbox for the anti-theft system.
I plugged it in with the driver's door shut; after it quieted up, I opened the driver's door...and it went off again. Shut the door, locked it with the key, unlocked it with the key, and opened it again...and no horn. So, going by what my manual says, it appears to be operating normally now.
However, I have one complaint...apparently, this is how it's supposed to function, but the horn will go off if I open the trunk, even with the master key, while the door locks are locked. Is there a way to bypass this while allowing unauthorized (i.e. breaking open) trunk openings to sound the alarm? I would never go and install a car alarm, but if the car already has it, I may as well use it.
Thanks!