Dont know this specific vehicle setup but to me it sounds like the an alarm is activating when you connect the battery cable.
This can annd does occur with aftermarket alarms all the time when reinstalling a battery cable.
Try this,when you install the cable and the horn honks,if you have a remote,click it and see if it stops....if no remote,then insert the key in the drivers door lock and turn it i believe left,right left or right left right ,something like that and see if it shuts off...
Maybe just turn it to unlock...this should work..
Let me know