Those that have replaced the siren have said it's just a speaker replacement. The speaker being the siren. It's merely the output for the alarm moduel. I would have to assume that its your key fob that continues to transmit after pushing the button or your alarm module being faulty. An easy test of the key fob is test the range of the fob. Press the button from 100' and get closer or farther to activate the alarm, once you activate it then walk away and see if it stops flashing. If so then it's in your key fob, if not then it's your alarm module.

From others that have posted different electrical issues relating to the trunk area. It all came back that the seal around the trunk area didn't seal properly and allowed water into the bottom of the trunk which houses several electrical modules. You may want to check this area for dampness or maybe correlate to when you washed your car or possibly was out in the rain?