hi sorry to hear about your mondeo.
if it was me i would lift the bonnet and you will see a black switch sticking up at the front, press that ten times rapidly, you will then hear a beep, quickly shut the bonnet and go around the car opening and closing the doors (one by one )
each time a door is open you should hear a beep, if you dont hear a beep then that door maybe faulty and might be setting off the alarm, dont forget to do the boot and bonnet aswell, and when you have done all that, leave the car for about 20-30 seconds( dont open any doors ) and it will reset and go back to normal, but make sure you dont have a butterfly or fly in the car as that can cause alarm to go off too. hope that helps, if not then disconnect the rear horn that you mentioned, or replace the alarm module( under passenger glove box..
this site will help you loads
http://www.mirez.co.uk/AlarmProbs.htm
regards
adrian